ID:394 - close fp if isn't NULL and set it NULL afterwards in USB plugin

Commit brings back check whether fp is or isn't NULL. If fp isn't NULL, close()
is called and fp is set to NULL afterwards.

src/plugins/usb/usb.c

@@ -147,7 +147,10 @@ scsiProbeNew(int *num_ami_devices, int *sg_nos)
 	}
 
 	*num_ami_devices = numdevfound;
+	if (fp != NULL) {
 		fclose(fp);
+		fp = NULL;
+	}
 
 	return 0;
 }