vim-patch:8.2.4245: ":retab 0" may cause illegal memory access

Problem:    ":retab 0" may cause illegal memory access.
Solution:   Limit the value of 'tabstop' to 10000.
652dee4486

ex_retab change is N/A (+vartabs always available).

Nvim's set_num_option validation logic was refactored, hence why it looks
different from Vim's.

Also use XFREE_CLEAR in other places.

src/nvim/option_defs.h

@@ -903,6 +903,8 @@ enum {
 
 #define SB_MAX 100000  // Maximum 'scrollback' value.
 
+#define TABSTOP_MAX 9999
+
 /// Stores an identifier of a script or channel that last set an option.
 typedef struct {
   sctx_T script_ctx;       /// script context where the option was last set