Fix usbmuxd_send returning positive value on error

Since socket_send already returns -errno, this function should pass it as is
diff --git a/src/libusbmuxd.c b/src/libusbmuxd.c
index 8647ee2..8329bef 100644
--- a/src/libusbmuxd.c
+++ b/src/libusbmuxd.c
@@ -1593,7 +1593,7 @@
 	if (num_sent < 0) {
 		*sent_bytes = 0;
 		LIBUSBMUXD_DEBUG(1, "%s: Error %d when sending: %s\n", __func__, -num_sent, strerror(-num_sent));
-		return -num_sent;
+		return num_sent;
 	}
 
 	if ((uint32_t)num_sent < len) {