csi: check for private marker in 'S' case

The handler for 'S' final character does not check for a private
marker. This can cause a conflict with a sequence called 'XTSMGRAPHICS'
which also has an 'S' final character, but uses the private marker '?'.
Without checking for a private marker, st will perform a scroll up
operation when XTSMGRAPHICS is seen, which can cause unexpected display
artifacts.

st.c

@@ -1728,6 +1728,7 @@ csihandle(void)
 		}
 		break;
 	case 'S': /* SU -- Scroll <n> line up */
+		if (csiescseq.priv) break;
 		DEFAULT(csiescseq.arg[0], 1);
 		tscrollup(term.top, csiescseq.arg[0]);
 		break;