Keymap: check modal keymap item poll function

Fixes T56591: Vertex/Edge slide C-Key fails

source/blender/windowmanager/intern/wm_event_system.c

@@ -1871,16 +1871,18 @@ static void wm_event_modalkeymap(const bContext *C, wmOperator *op, wmEvent *eve
 
 		for (kmi = keymap->items.first; kmi; kmi = kmi->next) {
 			if (wm_eventmatch(event, kmi)) {
-
+				if ((keymap->poll_modal_item == NULL) ||
+				    (keymap->poll_modal_item(op, kmi->propvalue)))
+				{
 					event->prevtype = event->type;
 					event->prevval = event->val;
 					event->type = EVT_MODAL_MAP;
 					event->val = kmi->propvalue;
-
 					break;
 				}
 			}
 		}
+	}
 	else {
 		/* modal keymap checking returns handled events fine, but all hardcoded modal
 		 * handling typically swallows all events (OPERATOR_RUNNING_MODAL).