revwalk: The left operand of '<' is a garbage value

At line 594, we do this :
if (error < 0)
		return error;

but if nothing was pushed in a GIT_SORT_TIME revwalk, we'd return
uninitialized stack data.

(cherry picked from commit aa8cb586)

src/revwalk.c

@@ -522,7 +522,7 @@ cleanup:
 
 static int prepare_walk(git_revwalk *walk)
 {
-	int error;
+	int error = 0;
 	git_commit_list *list, *commits = NULL;
 	git_commit_list_node *next;