Fix crash getting next frame in NPCs

Attempting to animate next frame for an NPC that is not doing the expected action would sometimes put the npc's frame out of bounds, which later caused a crash when attempting to load the texture for that (non-existent) frame.

EOLib/Domain/Extensions/NPCExtensions.cs

@@ -38,6 +38,9 @@ public static int GetWalkFrame(this NPC.NPC npc)
 
         public static NPC.NPC WithNextWalkFrame(this NPC.NPC npc)
         {
+            if (npc.IsActing(NPCActionState.Attacking))
+                return npc;
+
             if (npc.Frame == NPCFrame.WalkFrame4)
             {
                 return npc.WithFrame(NPCFrame.Standing);
@@ -52,6 +55,9 @@ public static NPC.NPC WithNextWalkFrame(this NPC.NPC npc)
 
         public static NPC.NPC WithNextAttackFrame(this NPC.NPC npc)
         {
+            if (npc.IsActing(NPCActionState.Walking))
+                return npc;
+
             var retNpc = npc.WithActualAttackFrame((npc.ActualAttackFrame + 1) % 5);
 
             if (npc.ActualAttackFrame == 0)