memory leak in tst_fills2.c

[edhartnett]

See #504 for build info.

./tst_fills2

*** Testing fill values.
*** testing read of string record var with no data...ok.
*** testing read of string record var w/fill-value with no data...ok.
*** testing NULL fill values of a string var...ok.
*** testing non-empty fill values of a string var...ok.
*** testing read of string record var with no data...ok.
*** Testing fix for netCDF-4 fill-value bug...ok.
*** Tests successful!

=================================================================
==1291==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 322 byte(s) in 7 object(s) allocated from:
   #0 0x7f57dcc72ec0 in __interceptor_malloc (/usr/lib/x86_64-linux-gnu/libasan.so.3+0xc6ec0)
   #1 0x7f57d827d55c in H5T_vlen_str_mem_write /home/ed/Downloads/hdf5-1.10.1/src/H5Tvlen.c:692

SUMMARY: AddressSanitizer: 322 byte(s) leaked in 7 allocation(s).