Fix access of first element of an empty vector in pynestkernel

[niltonlk]

This PR fix out of bound access in pynestkernel, reported in #2101.
With this fix, the two pynest examples (recording_demo.py and balancedneuron.py) reported in #2101 now succeed.

[jougs]

LGTM. I suggest a quick merge as this solves a real problem.

[hakonsbm]

Just a tiny typo.

[heplesser]

@niltonlk The failure is curious and likely related to using multiple test runners (if several tests write to files of the same name, we will get into trouble). Could you for now change --numprocesses=auto to --numprocesses=1 in these two places:

nest-simulator/testsuite/do_tests.sh

Line 102 in 12a897a

PYTEST_VERSION="$(${PYTHON} -m pytest --version --timeout ${TIME_LIMIT} --numprocesses=auto 2>&1)" || {

nest-simulator/testsuite/do_tests.sh

Line 473 in 12a897a

"${PYTHON}" -m pytest --verbose --timeout $TIME_LIMIT --junit-xml="${XUNIT_FILE}" --numprocesses=auto \

[heplesser]

I had another look at the code and am certain that nothing went wrong in the past, because for an empty vector_ptr

memcpy(array_data, &vector_ptr.front(), vector_ptr.size() * sizeof(vector_value_t));

would not have copied anything. But it is clearly incorrect to request front() of an empty container, to this fix is needed.

[hakonsbm]

Thanks! Looks good now.