saned: fix [-Wmaybe-uninitialized] compiler warning

frontend/saned.c

@@ -1431,7 +1431,7 @@ start_scan (Wire * w, int h, SANE_Start_Reply * reply)
   SANE_Handle be_handle;
   int fd, len;
   in_port_t data_port;
-  int ret;
+  int ret = -1;
 
   be_handle = handle[h].handle;