tests: Fix heap overflow in coredump-unwind test
tests: Fix heap overflow in coredump-unwind test

We've tried to run slightly modified test-coredump-unwind.c built with
tcmalloc, and it promptly crashed.  Attached patch fixes the heap buffer
overflow bug which caused it.

yuyichao/libunwind,vtjnash/libunwind,vtjnash/libunwind,frida/libunwind,rantala/libunwind,djwatson/libunwind,libunwind/libunwind,frida/libunwind,frida/libunwind,yuyichao/libunwind,vtjnash/libunwind,rantala/libunwind,yuyichao/libunwind,rantala/libunwind,djwatson/libunwind,libunwind/libunwind,libunwind/libunwind,djwatson/libunwind