use assert(0) instead of exit(1) (backport from master to v2)

When a fatal error (unaligned memory etc.) is detected, jerasure
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>
(cherry picked from commit 36008101d5fd1e3d19230828c0eb231869569893)