vim-patch:8.2.4151: reading beyond the end of a line

Problem:    Reading beyond the end of a line.
Solution:   For block insert only use the offset for correcting the length.
57df9e8a9f

src/nvim/ops.c

@@ -568,19 +568,8 @@ static void block_insert(oparg_T *oap, char_u *s, int b_insert, struct block_def
     }
 
     if (spaces > 0) {
-      int off;
+      // avoid copying part of a multi-byte character
-
+      offset -= utf_head_off(oldp, oldp + offset);
-      // Avoid starting halfway through a multi-byte character.
-      if (b_insert) {
-        off = utf_head_off(oldp, oldp + offset + spaces);
-        spaces -= off;
-        count -= off;
-      } else {
-        // spaces fill the gap, the character that's at the edge moves
-        // right
-        off = utf_head_off(oldp, oldp + offset);
-        offset -= off;
-      }
     }
     if (spaces < 0) {  // can happen when the cursor was moved
       spaces = 0;

src/nvim/testdir/test_visual.vim

@@ -1271,6 +1271,15 @@ func Test_visual_reselect_with_count()
   call delete('XvisualReselect')
 endfunc
 
+func Test_visual_block_insert_round_off()
+  new
+  " The number of characters are tuned to fill a 4096 byte allocated block,
+  " so that valgrind reports going over the end.
+  call setline(1, ['xxxxx', repeat('0', 1350), "\t", repeat('x', 60)])
+  exe "normal gg0\<C-V>GI" .. repeat('0', 1320) .. "\<Esc>"
+  bwipe!
+endfunc
+
 " this was leaving the end of the Visual area beyond the end of a line
 func Test_visual_ex_copy_line()
   new