Avoid pointless check before calling free

Signed-off-by: Mans Rullgard <mans@mansr.com>
(cherry picked from commit dc75d6db)

libavutil/mem.c

@@ -138,9 +138,8 @@ void *av_realloc(void *ptr, FF_INTERNAL_MEM_TYPE size)
 
 void av_free(void *ptr)
 {
-    /* XXX: this test should not be needed on most libcs */
-    if (ptr)
 #if CONFIG_MEMALIGN_HACK
+    if (ptr)
         free((char*)ptr - ((char*)ptr)[-1]);
 #else
     free(ptr);