Clear out `ip' to avoid -Wuninitialized warning in tests/test-coredump-unwind.c

This one is for architectures that we have not specifically added
support for in `tests/test-coredump-unwind.c'.

tests/test-coredump-unwind.c: In function 'handle_sigsegv':
test-coredump-unwind.c:238:10: warning: 'ip' is used uninitialized in this function [-Wuninitialized]
1 file changed