sparse-revlog: handle nullrev in index_get_length

sparse-revlog: handle nullrev in index_get_length

The more generic index_get method handle nullrev fine, we apply the same logic
here.

--- a/mercurial/cext/revlog.c	Fri Dec 14 17:23:41 2018 +0100
+++ b/mercurial/cext/revlog.c	Fri Dec 14 17:24:44 2018 +0100
@@ -261,6 +261,9 @@
 
 static inline int index_get_length(indexObject *self, Py_ssize_t rev)
 {
+	if (rev == nullrev) {
+		return 0;
+	}
 	if (rev >= self->length) {
 		PyObject *tuple;
 		PyObject *pylong;