Fix UndefinedBehaviourSanitizer errors.

[olupton]

This PR contains fixes for issues uncovered by the UndefinedBehaviour sanitizer: https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html.

units.cpp: index -1 out of bounds for type 'struct unit [20]'

A unit* pointer uspused the -1th element of an array as an invalid value. Fixed by using nullptr instead.

{path}/nrn/src/nmodl/../modlunit/units.cpp:569:19: runtime error: index -1 out of bounds for type 'struct unit [20]'
    #0 0x10fc3bfcf in unit_stk_clean() units.cpp:569
    #1 0x10fc3c7ed in unit_init() units.cpp:667
    #2 0x10fc3c069 in modl_units() units.cpp:607
    #3 0x10fb8aea8 in yyparse() parse1.ypp:1112
    #4 0x10fbdb9f7 in main modl.cpp:164
    #5 0x7fff20423f3c in start+0x0 (libdyld.dylib:x86_64+0x15f3c)

parallel_partrans test, null pointer arithmetic

{path}/nrn/src/nrniv/nonlinz.cpp:545:24: runtime error: applying non-zero offset 8 to null pointer
    #0 0x10178d03d in NonLinImpRep::current(int, Memb_list*, int) nonlinz.cpp:545
    #1 0x10177f71b in NonLinImpRep::didv() nonlinz.cpp:349
    #2 0x10177bc43 in NonLinImp::compute(double, double, int) nonlinz.cpp:148
    #3 0x101681cd3 in Imp::compute(double, bool, int) impedanc.cpp:265
    #4 0x101686917 in compute(void*) impedanc.cpp
    #5 0x101995f66 in hoc_call_ob_proc hoc_oop.cpp:759
    #6 0x10199913d in hoc_object_component() hoc_oop.cpp:1174
    #7 0x101dd135d in component(PyHocObject*) nrnpy_hoc.cpp:464
    #8 0x101dd03df in fcall(void*, void*) nrnpy_hoc.cpp:660
    #9 0x101837a7a in OcJumpImpl::fpycall(void* (*)(void*, void*), void*, void*) ocjump.cpp:222
    #10 0x101dcfe4d in hocobj_call(PyHocObject*, _object*, _object*) nrnpy_hoc.cpp:764
    #11 0x102a2ab13 in _PyObject_MakeTpCall+0x109 (Python:x86_64+0x32b13)
    #12 0x102ad3df2 in call_function+0x1c6 (Python:x86_64+0xdbdf2)
    #13 0x102ad1354 in _PyEval_EvalFrameDefault+0x6416 (Python:x86_64+0xd9354)
    #14 0x102a2b1b8 in function_code_fastcall+0x60 (Python:x86_64+0x331b8)
    #15 0x102ad3dbe in call_function+0x192 (Python:x86_64+0xdbdbe)
    #16 0x102ad13f2 in _PyEval_EvalFrameDefault+0x64b4 (Python:x86_64+0xd93f2)
    #17 0x102ad4897 in _PyEval_EvalCode+0x75e (Python:x86_64+0xdc897)
    #18 0x102acae85 in PyEval_EvalCode+0x38 (Python:x86_64+0xd2e85)
    #19 0x102b05a14 in run_eval_code_obj+0x6d (Python:x86_64+0x10da14)
    #20 0x102b04be8 in run_mod+0x66 (Python:x86_64+0x10cbe8)
    #21 0x102b04d66 in pyrun_file+0xd7 (Python:x86_64+0x10cd66)
    #22 0x102b033dd in PyRun_SimpleFileExFlags+0x2a3 (Python:x86_64+0x10b3dd)
    #23 0x101dcacc2 in nrnpython_start nrnpython.cpp:184
    #24 0x101451d97 in ivocmain_session(int, char const**, char const**, int) ivocmain.cpp:859
    #25 0x101451755 in ivocmain(int, char const**, char const**) ivocmain.cpp:415
    #26 0x10141da8f in main nrnmain.cpp:53
    #27 0x7fff20423f3c in start+0x0 (libdyld.dylib:x86_64+0x15f3c)

coming from https://github.com/neuronsimulator/nrn/blob/5873be8a580973b790d187a91b36102cb99a431d/src/nrniv/nonlinz.cpp#L545
fixed with mfake.prop = ml->prop ? ml->prop + in : nullptr;

Others

This is not comprehensive. #1479 and #1483 both describe additional issues.

[pramodk]

LGTM

(cc @nrnhines: just a note, we are splitting #1479 into two smaller PRs so that easy/smaller changes could be merged quicker)

[codecov-commenter]

Codecov Report

Merging #1518 (b4ceebc) into master (f4089c8) will increase coverage by 0.08%.
The diff coverage is 100.00%.

❗ Current head b4ceebc differs from pull request most recent head bfcc603. Consider uploading reports for the commit bfcc603 to get more accurate results

@@            Coverage Diff             @@
##           master    #1518      +/-   ##
==========================================
+ Coverage   42.86%   42.95%   +0.08%     
==========================================
  Files         550      550              
  Lines      110342   110346       +4     
==========================================
+ Hits        47301    47400      +99     
+ Misses      63041    62946      -95     

Impacted Files	Coverage Δ
src/modlunit/units.cpp	78.44% <100.00%> (+0.14%)	⬆️
src/nrniv/nonlinz.cpp	73.03% <100.00%> (ø)
share/lib/python/neuron/rxd/species.py	77.01% <0.00%> (+0.07%)	⬆️
src/parallel/bbs.cpp	64.55% <0.00%> (+1.68%)	⬆️
src/parallel/bbssrv2mpi.cpp	56.14% <0.00%> (+3.74%)	⬆️
src/nrnmpi/bbsmpipack.cpp	86.15% <0.00%> (+11.28%)	⬆️
src/parallel/bbsclimpi.cpp	50.58% <0.00%> (+22.09%)	⬆️
src/parallel/bbssrvmpi.cpp	44.57% <0.00%> (+27.71%)	⬆️

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