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)

src/regexp.c

@@ -2543,6 +2543,7 @@ reg_submatch_list(int no)
 	list_free(list);
 	return NULL;
     }
+    ++list->lv_refcount;
     return list;
 }
 #endif

src/testdir/test_substitute.vim

@@ -881,6 +881,12 @@ func Test_invalid_submatch()
   call assert_equal([], submatch(1, 1))
 endfunc
 
+func Test_submatch_list_concatenate()
+  let pat = 'A\(.\)'
+  let Rep = {-> string([submatch(0, 1)] + [[submatch(1)]])}
+  call substitute('A1', pat, Rep, '')->assert_equal("[['A1'], ['1']]")
+endfunc
+
 func Test_substitute_expr_arg()
   call assert_equal('123456789-123456789=', substitute('123456789',
 	\ '\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)\(.\)',

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1621,
 /**/
     1620,
 /**/