Meet 'assert_mem` like `check_mem` but fatal
include assert.h when using assert.
Update every check_mem to an assert_mem

This removed a LOT of error exits as well.
Lose the redundant assertions in bsp.c
Lose redundant assertions in export.c
Redundant assertion purge of poly.
Redundant memory assertion in stl_mesh.c
util.c: Remove redundant memory assertions
Merge pull request #58 from sshirokov/memory-is-fatal

Make running out of memory assert()