patch 8.2.2640: screenstring() returns non-existing composing characters

Problem:    screenstring() returns non-existing composing characters.
Solution:   Only use composing characters if there is a base character.

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2640,
 /**/
     2639,
 /**/