compositor: return failure even if SEGV handler works v2

compositor: return failure even if SEGV handler works v2

Weston has a SIGSEGV handler that attempts to shut everything down
cleanly. If it actually succeeds in that, the process exit status will
indicate success, when the process just segfaulted.

Fix that by setting the return value to failure in the SEGV handler.

v2: use a local instead of a global variable

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>