mem: Trying to workaround posix_memalign() bug on OSX

This patch is based on a patch by John Stebbins
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/mem.c

@@ -86,6 +86,7 @@ void *av_malloc(size_t size)
     ptr = (char*)ptr + diff;
     ((char*)ptr)[-1]= diff;
 #elif HAVE_POSIX_MEMALIGN
+    if (size) //OSX on SDK 10.6 has a broken posix_memalign implementation
     if (posix_memalign(&ptr,ALIGN,size))
         ptr = NULL;
 #elif HAVE_MEMALIGN