Fix berserker fidget animation aborting attack.

M_MoveFrame() calls first the AI functions that decide if a monster
should attack or not. After that the monsters think function is called
which walks through berserk_frames_stand[]. Even if the AI function found
an enemy and decided to attack, the monster is still standing for this
frame and berserker_fidget() is called. It may override the earlier
earlier decision, aborting the attack. Even worse this may let the
berserker stuck, because AI_STAND_GROUND may be cleared which prevents
further attacks.

This bug was present in the original code, so this is small gameplay
change. It's likely also present in both addons.

Reported and analyzed by @BjossiAlfreds in issue #433. He also suggested
the fix.

src/monster/berserker/berserker.c

@@ -103,6 +103,11 @@ berserk_fidget(edict_t *self)
 		return;
 	}
 
+	if (self->enemy)
+	{
+		return;
+	}
+
 	if (self->monsterinfo.aiflags & AI_STAND_GROUND)
 	{
 		return;