transport/http: do not return success if we failed to get a scheme

Otherwise we return a NULL context, which will get dereferenced in 
apply_credentials.

src/transports/http.c

@@ -142,7 +142,7 @@ static int auth_context_match(
 	}
 
 	if (!scheme)
-		return 0;
+		return -1;
 
 	/* See if authentication has already started for this scheme */
 	git_vector_foreach(&t->auth_contexts, i, c) {