Fixed limits for search space of object position. (#65)

* Fixed upper limit for search space of object position.

* Fixed lower limit for search space of object position.

gym_minigrid/minigrid.py

@@ -908,6 +908,8 @@ class MiniGridEnv(gym.Env):
 
         if top is None:
             top = (0, 0)
+        else:
+            top = (max(top[0], 0), max(top[1], 0))
 
         if size is None:
             size = (self.grid.width, self.grid.height)
@@ -923,8 +925,8 @@ class MiniGridEnv(gym.Env):
             num_tries += 1
 
             pos = np.array((
-                self._rand_int(top[0], top[0] + size[0]),
-                self._rand_int(top[1], top[1] + size[1])
+                self._rand_int(top[0], min(top[0] + size[0], self.grid.width)),
+                self._rand_int(top[1], min(top[1] + size[1], self.grid.height))
             ))
 
             # Don't place the object on top of another object