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.

src/testdir/test_visual.vim

@@ -1309,5 +1309,14 @@ 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
+
 
 " vim: shiftwidth=2 sts=2 expandtab