mem: Don't abort on av_malloc(0) in debug mode

This makes the behaviour consistent between debug and release mode.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavutil/mem.c

@@ -68,8 +68,6 @@ void *av_malloc(size_t size)
     long diff;
 #endif
 
-    assert(size);
-
     /* let's disallow possible ambiguous cases */
     if (size > (INT_MAX-32) || !size)
         return NULL;