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.
diff --git a/src/version.c b/src/version.c
index e2519f4..2c52166 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2640,
+/**/
     2639,
 /**/
     2638,