Backslide against walls (fixes #1914)

* Backslide animation is not shown when the player is against a wall.
* SP is still consumed as normal.
Thanks to @zackdreaver!

src/map/skill.c

@@ -7937,8 +7937,8 @@ int skill_castend_nodamage_id (struct block_list *src, struct block_list *bl, ui
 		break;
 
 	case TF_BACKSLIDING: //This is the correct implementation as per packet logging information. [Skotlex]
-		clif_skill_nodamage(src,bl,skill_id,skill_lv,1);
-		skill_blown(src,bl,skill_get_blewcount(skill_id,skill_lv),unit_getdir(bl),BLOWN_IGNORE_NO_KNOCKBACK);
+		if (skill_blown(src,bl,skill_get_blewcount(skill_id,skill_lv),unit_getdir(bl),BLOWN_IGNORE_NO_KNOCKBACK))
+			clif_skill_nodamage(src, bl, skill_id, skill_lv, 1);
 		break;
 
 	case TK_HIGHJUMP: