Fix leaks with test_fuzz(..)

• Prevent a segfault from occurring if allocating rawimage failed.
• tmp was leaked if realloc(3) failed; free it and NULL it out to avoid a use-after-free.
• Don't leak rawimage if size == 0.
• Remove useless assert for image right before checking that it was NULL to fix some deadcode in the MK_ASSERT_DEBUG != no case.

CID: 1361651, 1361669, 1361679
Reported by: Coverity