CVE-2023-1170: at the end of do_put() (ops.c), clamp the cursor column
back to the line length (and recompute coladd under virtualedit=all)
so a Visual block put past the new NUL no longer leaves the cursor
pointing past end-of-line.
CVE-2023-1175: in op_yank() (ops.c), when the yank starts inside a
multi-cell tab under virtual editing, clamp bd.startspaces to 0
before incrementing startcol so a negative width can't drive the
later block-yank copy out of bounds.
CVE-2023-1264: in vgetorpeek() (getchar.c), guard the dead-character
putcmdline()/unputcmdline() paths with cmdlinehasbuffer() (new
ex_getln.c helper) so a nested :open command with a NULL ccline.cmdbuff
no longer dereferences it.