gatt: add return value check of io_get_fd() to sock_io_send()

It is necessary to add a return value check.

Found with the SVACE static analysis tool.
diff --git a/src/gatt-database.c b/src/gatt-database.c
index 8472aac..6c84b08 100644
--- a/src/gatt-database.c
+++ b/src/gatt-database.c
@@ -2630,6 +2630,7 @@
 {
 	struct msghdr msg;
 	struct iovec iov;
+	int fd;
 
 	iov.iov_base = (void *) data;
 	iov.iov_len = len;
@@ -2638,7 +2639,13 @@
 	msg.msg_iov = &iov;
 	msg.msg_iovlen = 1;
 
-	return sendmsg(io_get_fd(io), &msg, MSG_NOSIGNAL);
+	fd = io_get_fd(io);
+	if (fd < 0) {
+		error("io_get_fd() returned %d\n", fd);
+		return fd;
+	}
+
+	return sendmsg(fd, &msg, MSG_NOSIGNAL);
 }
 
 static void att_disconnect_cb(int err, void *user_data)