Fix a strict-overflow warning

Compiler -- gcc (Debian 7.3.0-5) 7.3.0

./libvpx/vp9/encoder/vp9_denoiser.c:374:9: assuming signed overflow
does not occur when assuming that (X + c) < X is always false
[-Wstrict-overflow]
         for (j = 0; j < xmis; j++) {

Change-Id: Ib7397e718ff717bdabc088fc4c6e1771381fb522

vp9/encoder/vp9_denoiser.c

@@ -379,7 +379,7 @@ void vp9_denoiser_denoise(VP9_COMP *cpi, MACROBLOCK *mb, int mi_row, int mi_col,
           // zero/small motion in skin detection is high, i.e, > 4).
           if (consec_zeromv < 4) {
             i = ymis;
-            j = xmis;
+            break;
           }
         }
       }