use assert(0) instead of exit(1)

When a fatal error (unaligned memory etc.) is detected, gf-complete should
assert(3) instead of exit(3) to give a chance to the calling program to
catch the exception and display a stack trace. Although it is possible
for gdb to display the stack trace and break on exit, libraries are not
usually expected to terminate the calling program in this way.

Signed-off-by: Loic Dachary <loic@dachary.org>