smart_pkt: treat empty packet lines as error

The Git protocol does not specify what should happen in the case
of an empty packet line (that is a packet line "0004"). We
currently indicate success, but do not return a packet in the
case where we hit an empty line. The smart protocol was not
prepared to handle such packets in all cases, though, resulting
in a `NULL` pointer dereference.

Fix the issue by returning an error instead. As such kind of
packets is not even specified by upstream, this is the right
thing to do.
diff --git a/src/transports/smart_pkt.c b/src/transports/smart_pkt.c
index b1eee3e..e05196c 100644
--- a/src/transports/smart_pkt.c
+++ b/src/transports/smart_pkt.c
@@ -437,12 +437,13 @@
 	line += PKT_LEN_SIZE;
-	 * TODO: How do we deal with empty lines? Try again? with the next
-	 * line?
+	 * The Git protocol does not specify empty lines as part
+	 * of the protocol. Not knowing what to do with an empty
+	 * line, we should return an error upon hitting one.
 	if (len == PKT_LEN_SIZE) {
-		*out = line;
-		return 0;
+		giterr_set_str(GITERR_NET, "Invalid empty packet");
+		return GIT_ERROR;
 	if (len == 0) { /* Flush pkt */