avutil/mem: Fix flipped condition

Fixes return code and later null pointer dereference
Found-by: Laurent Butti <laurentb@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/mem.c

@@ -191,7 +191,7 @@ int av_reallocp_array(void *ptr, size_t nmemb, size_t size)
 {
     void **ptrptr = ptr;
     *ptrptr = av_realloc_f(*ptrptr, nmemb, size);
-    if (!*ptrptr && !(nmemb && size))
+    if (!*ptrptr && nmemb && size)
         return AVERROR(ENOMEM);
     return 0;
 }