Fixes possible crash if url is malicious

Try scan a QR code with content `ss://sdf invalid`, an uncaught
exception raises.

ShadowsocksX-NG/Utils.m

@@ -56,7 +56,10 @@ void ScanQRCodeOnScreen() {
             NSLog(@"%@", feature.messageString);
             if ( [feature.messageString hasPrefix:@"ss://"] )
             {
-                [foundSSUrls addObject:[NSURL URLWithString:feature.messageString]];
+                NSURL *url = [NSURL URLWithString:feature.messageString];
+                if (url) {
+                    [foundSSUrls addObject:url];
+                }
             }
         }
          CGImageRelease(image);