vim-patch:9.1.0752: can set 'cedit' to an invalid value (#30616)

Problem:  can set cedit to an invalid value
Solution: Check that the value is a valid key name
          (Milly)

closes: vim/vim#15778

25732435c5

Co-authored-by: Milly <milly.ca@gmail.com>

src/nvim/ex_getln.c

@@ -4302,7 +4302,7 @@ const char *did_set_cedit(optset_T *args)
     cedit_key = -1;
   } else {
     int n = string_to_key(p_cedit);
-    if (vim_isprintc(n)) {
+    if (n == 0 || vim_isprintc(n)) {
       return e_invarg;
     }
     cedit_key = n;