Use correct stride in set_planes_to_neutral_grey

Use the correct stride for both the Y and UV planes in
set_planes_to_neutral_grey().

The fix was suggested by lingkun_cheng@apple.com.

Bug: aomedia:3255
Change-Id: I3c3f954f986427f8561ee9dc6109d03f298c061a
diff --git a/av1/decoder/decodeframe.c b/av1/decoder/decodeframe.c
index 494ef0e..291974a 100644
--- a/av1/decoder/decodeframe.c
+++ b/av1/decoder/decodeframe.c
@@ -109,7 +109,7 @@
     for (int plane = only_chroma; plane < MAX_MB_PLANE; plane++) {
       const int is_uv = plane > 0;
       for (int row_idx = 0; row_idx < buf->crop_heights[is_uv]; row_idx++) {
-        memset(&buf->buffers[plane][row_idx * buf->uv_stride], 1 << 7,
+        memset(&buf->buffers[plane][row_idx * buf->strides[is_uv]], 1 << 7,
                buf->crop_widths[is_uv]);
       }
     }