patch 8.2.2469: confusing error if :winsize has a wrong argument

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

Author: brammool

src/ex_docmd.c

@@ -7347,6 +7347,11 @@ ex_winsize(exarg_T *eap)
     char_u	*arg = eap->arg;
     char_u	*p;
 
+    if (!isdigit(*arg))
+    {
+	semsg(_(e_invarg2), arg);
+	return;
+    }
     w = getdigits(&arg);
     arg = skipwhite(arg);
     p = arg;

src/testdir/test_excmd.vim

@@ -327,6 +327,9 @@ endfunc
 " Test for the :winsize command
 func Test_winsize_cmd()
   call assert_fails('winsize 1', 'E465:')
+  call assert_fails('winsize 1 x', 'E465:')
+  call assert_fails('win_getid(1)', 'E475: Invalid argument: _getid(1)')
+  " Actually changing the window size would be flaky.
 endfunc
 
 " Test for the :redir command

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2469,
 /**/
     2468,
 /**/