Add missing cast in RTF paste data construction.

udata[uindex] is a wchar_t, so if we pass it to sprintf("%d") we
should cast it to int (because who knows what primitive integer type
that might have corresponded to otherwise). I had done this in the
first of the two sprintfs that use it, but missed the second one a few
lines further on. Spotted by Coverity.

windows/window.c

@@ -5255,7 +5255,7 @@ static void wintw_clip_write(
                                    (int)udata[uindex]);
                     alen = 1; strcpy(after, "}");
                 } else {
-                    blen = sprintf(before, "\\u%d", udata[uindex]);
+                    blen = sprintf(before, "\\u%d", (int)udata[uindex]);
                     alen = 0; after[0] = '\0';
                 }
             }