lavu/internal: Replace an empty loop with "do {}".

Silences a clang warning when not compiling for x86:
libswscale/utils.c:345:13: warning: while loop has empty body

Suggested-by: Nicolas George

libavutil/internal.h

@@ -52,7 +52,7 @@
 #endif
 
 #ifndef emms_c
-#   define emms_c() while(0)
+#   define emms_c() do {} while(0)
 #endif
 
 #ifndef attribute_align_arg