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/regexp.c b/src/regexp.c
index 229f6ef..375731c 100644
--- a/src/regexp.c
+++ b/src/regexp.c
@@ -2543,6 +2543,7 @@
 	list_free(list);
 	return NULL;
     }
+    ++list->lv_refcount;
     return list;
 }
 #endif