patch 8.2.2504: Vim9: crash when using an argument from a closure

Problem:    Vim9: crash when using an argument from a closure.
Solution:   Check if gen_load_outer is NULL. (closes #7821)

src/testdir/test_vim9_func.vim

@@ -1669,6 +1669,18 @@ def Test_closure_using_argument()
 
   unlet g:UseArg
   unlet g:UseVararg
+
+  var lines =<< trim END
+      vim9script
+      def Test(Fun: func(number): number): list<number>
+        return map([1, 2, 3], (_, i) => Fun(i))
+      enddef
+      def Inc(nr: number): number
+        return nr + 2
+      enddef
+      assert_equal([3, 4, 5], Test(Inc))
+  END
+  CheckScriptSuccess(lines)
 enddef
 
 def MakeGetAndAppendRefs()

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2504,
 /**/
     2503,
 /**/

src/vim9compile.c

@@ -261,7 +261,8 @@ arg_exists(
 	if (arg_exists(name, len, idxp, type, gen_load_outer, cctx->ctx_outer)
 									 == OK)
 	{
-	    ++*gen_load_outer;
+	    if (gen_load_outer != NULL)
+		++*gen_load_outer;
 	    return OK;
 	}
     }