sanm: fix undefined behaviour on big-endian.

A variable with post-increment may only appear
once in a statement.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>

libavcodec/sanm.c

@@ -1044,8 +1044,10 @@ static int decode_5(SANMVideoContext *ctx)
 #if HAVE_BIGENDIAN
     npixels = ctx->npixels;
     frm = ctx->frm0;
-    while (npixels--)
-        *frm++ = av_bswap16(*frm);
+    while (npixels--) {
+        *frm = av_bswap16(*frm);
+        frm++;
+    }
 #endif
 
     return 0;