patch 8.2.1621: crash when using submatch(0, 1) in substitute()

Problem:    Crash when using submatch(0, 1) in substitute().
Solution:   Increment reference count. (closes #6887)
diff --git a/src/version.c b/src/version.c
index 36416ef..cf7de76 100644
--- a/src/version.c
+++ b/src/version.c
@@ -755,6 +755,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1621,
+/**/
     1620,
 /**/
     1619,