mem: Consistently return NULL for av_malloc(0)

Plain POSIX malloc(0) is allowed to return either NULL or a
non-NULL pointer. The calling code should be ready to handle
a NULL return as a correct return (instead of a failure) if the size
to allocate was 0 - this makes sure the condition is handled
in a consistent way across platforms.

This also avoids calling posix_memalign(&ptr, 32, 0) on OS X,
which returns an invalid pointer (a non-NULL pointer that causes
crashes when passed to av_free).

Abort in debug mode, to help track down issues related to
incorrect handling of this case.

Signed-off-by: Martin Storsjö <martin@martin.st>
