vp8: add do { } while(0) around XCHG() macro to avoid confusing if/else

This is the correct solution to the warning "fixed" in the previous
commit.

Originally committed as revision 24367 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/vp8.c

@@ -882,16 +882,17 @@ void xchg_mb_border(uint8_t *top_border, uint8_t *src_y, uint8_t *src_cb, uint8_
     src_cb -= uvlinesize;
     src_cr -= uvlinesize;
 
-#define XCHG(a,b,xchg)\
+#define XCHG(a,b,xchg) do {\
 if (xchg) AV_SWAP64(b,a);\
-else      AV_COPY64(b,a);
+else      AV_COPY64(b,a);\
+} while (0)
 
     XCHG(top_border_m1+8, src_y-8, xchg);
     XCHG(top_border,      src_y,   xchg);
     XCHG(top_border+8,    src_y+8, 1);
-    if (mb_x < mb_width-1) {
+    if (mb_x < mb_width-1)
         XCHG(top_border+32, src_y+16, 1);
-    }
+
     // only copy chroma for normal loop filter
     // or to initialize the top row to 127
     if (!simple || !mb_y) {