patch 9.0.1611: v:maxcol can be changed in a :for loop

Problem:    v:maxcol can be changed in a :for loop.
Solution:   Check for read-only loop variable. (closes #12470)

src/evalvars.c

@@ -3842,7 +3842,8 @@ set_var_const(
 	    }
 
 	    if ((flags & ASSIGN_FOR_LOOP) == 0
-				 && var_check_permission(di, name) == FAIL)
+				 ? var_check_permission(di, name) == FAIL
+				 : var_check_ro(di->di_flags, name, FALSE))
 		goto failed;
 	}
 	else

src/testdir/test_eval_stuff.vim

@@ -126,6 +126,7 @@ func Test_for_invalid()
   call assert_fails("for x in 99", 'E1098:')
   call assert_fails("for x in function('winnr')", 'E1098:')
   call assert_fails("for x in {'a': 9}", 'E1098:')
+  call assert_fails("for v:maxcol in range(1)", 'E46:')
 
   if 0
     /1/5/2/s/\n

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1611,
 /**/
     1610,
 /**/