Fix crashes with mobs below or above due division by zero

* removes a division by zero issue which returns nan for yaw
  in some places

mods/mobs/api.lua

@@ -638,7 +638,14 @@ end
 function mobs:face_pos(self,pos)
 	local s = self.object:getpos()
 	local vec = {x=pos.x-s.x, y=pos.y-s.y, z=pos.z-s.z}
-	local yaw = math.atan(vec.z/vec.x)+math.pi/2
+	local yaw = 0
+	if vec.x ~= 0 then
+		yaw=math.atan(vec.z/vec.x)+math.pi/2
+	else
+		if vec.z>0 then
+			yaw=math.pi
+		end
+	end
 	if self.drawtype == "side" then
 		yaw = yaw+(math.pi/2)
 	end

mods/mobs/step.lua

@@ -144,7 +144,14 @@ function mobs.on_step(self,dtime)
 		end
 		
 		local vec = {x=p.x-s.x, y=p.y-s.y, z=p.z-s.z}
-		local yaw = math.atan(vec.z/vec.x)+math.pi/2
+		local yaw = 0
+		if vec.x ~= 0 then
+			yaw=math.atan(vec.z/vec.x)+math.pi/2
+		else
+			if vec.z>0 then
+				yaw=math.pi
+			end
+		end
 		if self.drawtype == "side" then
 			yaw = yaw+(math.pi/2)
 		end
@@ -223,7 +230,13 @@ function mobs.on_step(self,dtime)
 		end
 		
 		local vec = {x=p.x-s.x, y=p.y-s.y, z=p.z-s.z}
-		local yaw = math.atan(vec.z/vec.x)+math.pi/2
+		if vec.x ~= 0 then
+			yaw=math.atan(vec.z/vec.x)+math.pi/2
+		else
+			if vec.z>0 then
+				yaw=math.pi
+			end
+		end
 		if self.drawtype == "side" then
 			yaw = yaw+(math.pi/2)
 		end
@@ -345,7 +358,13 @@ function mobs.on_step(self,dtime)
 				self.v_start = false
 			else
 				local vec = {x=p.x-s.x, y=p.y-s.y, z=p.z-s.z}
-				local yaw = math.atan(vec.z/vec.x)+math.pi/2
+				if vec.x ~= 0 then
+					yaw=math.atan(vec.z/vec.x)+math.pi/2
+				else
+					if vec.z>0 then
+						yaw=math.pi
+					end
+				end
 				if self.drawtype == "side" then
 					yaw = yaw+(math.pi/2)
 				end