Have selected() check whether selection exists

Signed-off-by: Devin J. Pohly <djpohly@gmail.com>

st.c

@@ -419,7 +419,8 @@ selnormalize(void)
 int
 selected(int x, int y)
 {
-	if (sel.mode == SEL_EMPTY)
+	if (sel.mode == SEL_EMPTY || sel.ob.x == -1 ||
+			sel.alt != IS_SET(MODE_ALTSCREEN))
 		return 0;
 
 	if (sel.type == SEL_RECTANGULAR)