vim-patch:8.2.2469: confusing error if :winsize has a wrong argument (#13889)

Problem:    Confusing error if :winsize has a wrong argument.
Solution:   Quote the argument in the error. (closes vim/vim#2523)
f5a5116a96

Cherry-pick Test_winsize_cmd() from patch v8.2.0243.

src/nvim/ex_docmd.c

@@ -7756,6 +7756,11 @@ static void do_exmap(exarg_T *eap, int isabbrev)
 static void ex_winsize(exarg_T *eap)
 {
   char_u *arg = eap->arg;
+
+  if (!ascii_isdigit(*arg)) {
+    EMSG2(_(e_invarg2), arg);
+    return;
+  }
   int w = getdigits_int(&arg, false, 10);
   arg = skipwhite(arg);
   char_u *p = arg;