patch 8.2.4219: reading before the start of the line

Problem:    Reading before the start of the line.
Solution:   Check boundary before trying to read the character.

src/register.c

@@ -1474,7 +1474,7 @@ yank_copy_line(struct block_def *bd, long y_idx, int exclude_trailing_space)
     {
 	int s = bd->textlen + bd->endspaces;
 
-	while (VIM_ISWHITE(*(bd->textstart + s - 1)) && s > 0)
+	while (s > 0 && VIM_ISWHITE(*(bd->textstart + s - 1)))
 	{
 	    s = s - (*mb_head_off)(bd->textstart, bd->textstart + s - 1) - 1;
 	    pnew--;

src/testdir/test_visual.vim

@@ -1247,6 +1247,13 @@ func Test_visual_put_blockedit_zy_and_zp()
   bw!
 endfunc
 
+func Test_visual_block_yank_zy()
+  new
+  " this was reading before the start of the line
+  exe "norm o\<C-T>\<Esc>\<C-V>zy"
+  bwipe!
+endfunc
+
 func Test_visual_block_with_virtualedit()
   CheckScreendump
 

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4219,
 /**/
     4218,
 /**/