Fix error in FindScreenOfXY

In the previous commit, I defined the m_min variable both inside
and outside of an if block (forgot to delete the inside one). So
delete the extra definition. Also set m_min to the old return of
RB_MIN just in case (this also suppresses warnings from my complier).

libs/FScreen.c

@@ -824,13 +824,12 @@ FindScreenOfXY(int x, int y)
 			return (m);
 	}
 
-	struct monitor *m_min;
+	struct monitor *m_min = NULL;
 	if (m == NULL) {
 		/* No monitor found, point is in a dead area.
 		 * Find closest monitor using the taxi cab metric.
 		 */
 		int dmin = INT_MAX;
-		struct monitor *m_min;
 
 		RB_FOREACH(m, monitors, &monitor_q) {
 			int d = 0;