avcodec/pixlet: Reorder rlen check

This changes nothing but is nicer looking as this checks rlen

Maybe this helps coverity remove CID1397743
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/pixlet.c

@@ -173,7 +173,7 @@ static int read_low_coeffs(AVCodecContext *avctx, int16_t *dst, int size, int wi
             }
         }
 
-        if (i + rlen > size)
+        if (rlen > size - i)
             return AVERROR_INVALIDDATA;
         i += rlen;