stop pan key event even when pan anim is in progress (Leaflet#6231)

When pressing pan key multiple times while pan animation is in progress, `stop` function isn't invoked.
This led to document scroll (if document is scrollable)

This commit fixes the issue to stop pan key event even when pan animation is in progress.

src/map/handler/Map.Keyboard.js

@@ -145,20 +145,18 @@ export var Keyboard = Handler.extend({
 		    offset;
 
 		if (key in this._panKeys) {
+			if (!map._panAnim || !map._panAnim._inProgress) {
+				offset = this._panKeys[key];
+				if (e.shiftKey) {
+					offset = toPoint(offset).multiplyBy(3);
+				}
 
-			if (map._panAnim && map._panAnim._inProgress) { return; }
+				map.panBy(offset);
 
-			offset = this._panKeys[key];
-			if (e.shiftKey) {
-				offset = toPoint(offset).multiplyBy(3);
+				if (map.options.maxBounds) {
+					map.panInsideBounds(map.options.maxBounds);
+				}
 			}
-
-			map.panBy(offset);
-
-			if (map.options.maxBounds) {
-				map.panInsideBounds(map.options.maxBounds);
-			}
-
 		} else if (key in this._zoomKeys) {
 			map.setZoom(map.getZoom() + (e.shiftKey ? 3 : 1) * this._zoomKeys[key]);