Don't try to render tiles with nonsensical coordinates.

I'm not sure where these came from, but I had some invalid tiles kill the
renderers.  Now those tiles will be ignored.

queuemaster.py

@@ -511,6 +511,9 @@ def send_render_requests(self):
             renderer.send_request()
 
     def handle_render_request(self, t, props):
+        if not t.is_valid:
+            log_message('ignoring request for invalid tile: %s' % t)
+            return
         if t.exists(REFERENCE_TILESET):
             importance = 'important'
         else:

toposm.py

@@ -141,6 +141,15 @@ def is_old(self):
         else:
             return tileIsOld(self.z, self.x, self.y)
 
+    @property
+    def is_valid(self):
+        if self.is_metatile:
+            return 0 <= self.x and self.x < 2**self.z / NTILES[self.z] and \
+                   0 <= self.y and self.y < 2**self.z / NTILES[self.z]
+        else:
+            return 0 <= self.x and self.x < 2**self.z and \
+                   0 <= self.y and self.y < 2**self.z
+
 
 def getCachedMetaTileDir(mapname, z, x):
     return path.join(TEMPDIR, mapname, str(z), str(x))