mem: fix memalign hack av_realloc()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/mem.c

@@ -137,7 +137,9 @@ void *av_realloc(void *ptr, size_t size)
     //FIXME this isn't aligned correctly, though it probably isn't needed
     if(!ptr) return av_malloc(size);
     diff= ((char*)ptr)[-1];
-    return (char*)realloc((char*)ptr - diff, size + diff) + diff;
+    ptr= realloc((char*)ptr - diff, size + diff);
+    if(ptr) ptr = (char*)ptr + diff;
+    return ptr;
 #else
     return realloc(ptr, size + !size);
 #endif