decodframe.c: use vpx_memset instead of cast

Fix warning with -Wstrict-aliasing=1

Change-Id: Ic4e511af924247800e87d5f990636565f3dd91d2

vp8/decoder/decodframe.c

diff --git a/vp8/decoder/decodframe.c b/vp8/decoder/decodframe.c
index 50ee9c5..16da78a 100644
--- a/vp8/decoder/decodframe.c
+++ b/vp8/decoder/decodframe.c
@@ -211,7 +211,7 @@
                             (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]));
                     }
                 }
             }
@@ -248,21 +248,14 @@
 
                     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