patch 9.1.0021: i_CTRL-R- doesn't work for multibyte chars in Replace mode

Problem:  i_CTRL-R- doesn't work for multibyte chars in Replace mode,
          Coverity complains missing return value for u_save_cursor()
Solution: Use mb_charlen() and del_chars() instead, handle failure mode
          for u_save_cursor() correctly (@zeertzjq)

closes: #13846

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/register.c

@@ -832,8 +832,9 @@ insert_reg(
 		    if ((State & REPLACE_FLAG) != 0)
 		    {
 			pos_T curpos;
-			u_save_cursor();
+			if (u_save_cursor() == FAIL)
-			del_bytes((long)STRLEN(y_current->y_array[0]), TRUE, FALSE);
+			    return FAIL;
+			del_chars((long)mb_charlen(y_current->y_array[0]), TRUE);
 			curpos = curwin->w_cursor;
 			if (oneright() == FAIL)
 			    // hit end of line, need to put forward (after the current position)

src/testdir/test_registers.vim

@@ -963,6 +963,18 @@ func Test_insert_small_delete_replace_mode()
   let @- = ''
   exe ":norm! R\<C-R>-ZZZ"
   call assert_equal(['ZZZ', 'foofoo', '',  'bfoobarZZZZ'], getline(1, 4))
+  let @- = 'βbβ'
+  call cursor(4, 1)
+  exe ":norm! R\<C-R>-"
+  call assert_equal(['ZZZ', 'foofoo', '',  'βbβobarZZZZ'], getline(1, 4))
+  let @- = 'bβb'
+  call cursor(4, 1)
+  exe ":norm! R\<C-R>-"
+  call assert_equal(['ZZZ', 'foofoo', '',  'bβbobarZZZZ'], getline(1, 4))
+  let @- = 'βbβ'
+  call cursor(4, 1)
+  exe ":norm! R\<C-R>-"
+  call assert_equal(['ZZZ', 'foofoo', '',  'βbβobarZZZZ'], getline(1, 4))
   bwipe!
 endfunc
 

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    21,
 /**/
     20,
 /**/