do not show roster out of screen

src/roster_window.py

@@ -2040,8 +2040,13 @@ _('If "%s" accepts this request you will know his status.') %jid)
 		# no need of this variable
 		self.have_new_message_accel = False # Is the "Ctrl+N" shown ?
 		if gajim.config.get('saveposition'):
-			self.window.move(gajim.config.get('roster_x-position'),
-				gajim.config.get('roster_y-position'))
+			x = gajim.config.get('roster_x-position')
+			y = gajim.config.get('roster_y-position')
+			if x < 0:
+				x = 0
+			if y < 0:
+				y = 0
+			self.window.move(x, y)
 			self.window.resize(gajim.config.get('roster_width'),
 				gajim.config.get('roster_height'))