patch 8.2.3756: might crash when callback is not valid

Problem:    might crash when callback is not valid.
Solution:   Check for valid callback. (Yegappan Lakshmanan, closes #9293)
diff --git a/src/version.c b/src/version.c
index 294547b..c586acb 100644
--- a/src/version.c
+++ b/src/version.c
@@ -754,6 +754,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    3756,
+/**/
     3755,
 /**/
     3754,