avcodec/h264_mp4toannexb_bsf: Reorder operations in nal_size check

Fixes Ticket4778
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h264_mp4toannexb_bsf.c

@@ -201,7 +201,7 @@ static int h264_mp4toannexb_filter(AVBitStreamFilterContext *bsfc,
         buf      += ctx->length_size;
         unit_type = *buf & 0x1f;
 
-        if (buf + nal_size > buf_end || nal_size < 0)
+        if (nal_size > buf_end - buf || nal_size < 0)
             goto fail;
 
         if (unit_type == 7)