Add missing asserts in the FOV map functions

We were only checking that the x and y were greater or equal to zero,
but they must also be lower than the map's width and height.

Discovered this when my roguelike crashed because of this (assert within
libtcod itself).

src/map.rs

@@ -28,6 +28,8 @@ impl Map {
 
     pub fn set(&mut self, x: i32, y: i32, transparent: bool, walkable: bool) {
         assert!(x >= 0 && y >= 0);
+        let (width, height) = self.size();
+        assert!(x < width && y < height);
         unsafe {
             ffi::TCOD_map_set_properties(self.tcod_map, x, y,
                                          transparent as c_bool,
@@ -47,13 +49,17 @@ impl Map {
 
     pub fn is_in_fov(&self, x: i32, y: i32) -> bool {
         assert!(x >= 0 && y >= 0);
+        let (width, height) = self.size();
+        assert!(x < width && y < height);
         unsafe {
             ffi::TCOD_map_is_in_fov(self.tcod_map, x, y) != 0
         }
     }
 
     pub fn is_walkable(&self, x: i32, y: i32) -> bool {
         assert!(x >= 0 && y >= 0);
+        let (width, height) = self.size();
+        assert!(x < width && y < height);
         unsafe {
             ffi::TCOD_map_is_walkable(self.tcod_map, x, y) != 0
         }