threading.c: use vpx_memset instead of cast

Fix warning with -Wstrict-aliasing=1

Change-Id: I43df2781656dd7b3f4c0ec6daa283d7a78928a0c

vp8/decoder/threading.c

diff --git a/vp8/decoder/threading.c b/vp8/decoder/threading.c
index 7303189..fe290cf 100644
--- a/vp8/decoder/threading.c
+++ b/vp8/decoder/threading.c
@@ -227,7 +227,7 @@
                     {
                         vp8_dc_only_idct_add(b->qcoeff[0] * DQC[0],
                                              dst, dst_stride, dst, dst_stride);
-                        ((int *)b->qcoeff)[0] = 0;
+                        vpx_memset(b->qcoeff, 0, 2 * sizeof(b->qcoeff[0]));
                     }
                 }
             }
@@ -264,21 +264,14 @@
 
                     vp8_short_inv_walsh4x4(&b->dqcoeff[0],
                         xd->qcoeff);
-                    ((int *)b->qcoeff)[0] = 0;
-                    ((int *)b->qcoeff)[1] = 0;
-                    ((int *)b->qcoeff)[2] = 0;
-                    ((int *)b->qcoeff)[3] = 0;
-                    ((int *)b->qcoeff)[4] = 0;
-                    ((int *)b->qcoeff)[5] = 0;
-                    ((int *)b->qcoeff)[6] = 0;
-                    ((int *)b->qcoeff)[7] = 0;
+                    vpx_memset(b->qcoeff, 0, 16 * sizeof(b->qcoeff[0]));
                 }
                 else
                 {
                     b->dqcoeff[0] = b->qcoeff[0] * xd->dequant_y2[0];
                     vp8_short_inv_walsh4x4_1(&b->dqcoeff[0],
                         xd->qcoeff);
-                    ((int *)b->qcoeff)[0] = 0;
+                    vpx_memset(b->qcoeff, 0, 2 * sizeof(b->qcoeff[0]));
                 }
 
                 /* override the dc dequant constant in order to preserve the