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.
diff --git a/src/vim.h b/src/vim.h
index 15ed626..86acb6d 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -2085,6 +2085,8 @@
 
 #define DICT_MAXNEST 100	// maximum nesting of lists and dicts
 
+#define TABSTOP_MAX 9999
+
 #ifdef FEAT_CLIPBOARD
 
 // VIM_ATOM_NAME is the older Vim-specific selection type for X11.  Still