png: use av_mallocz_array() for the zlib zalloc function

Fixes valgrind uninitialized memory errors when decoding png.

CC:libav-stable@libav.org

libavcodec/png.c

@@ -47,9 +47,7 @@ const uint8_t ff_png_pass_mask[NB_PASSES] = {
 
 void *ff_png_zalloc(void *opaque, unsigned int items, unsigned int size)
 {
-    if(items >= UINT_MAX / size)
-        return NULL;
-    return av_malloc(items * size);
+    return av_mallocz_array(items, size);
 }
 
 void ff_png_zfree(void *opaque, void *ptr)