fix heap-buffer-overflow in mp4read.c

This originated from an integer overflow: If mp4config.frame.ents
would be read-in with a value of (uint32t)(-1), it would overflow to 0
in the size calculation for the allocation in the next line. The
malloc() function would then successfully return a pointer to a memory
region of size 0, which will cause a segfault when written to.

Fixes #57.