index: remove side-effect from failed nt_init()

As pointed out by Yuya in the review of D4108, if we run into the
"overflow in nt_init" case (which I think normally happens only in
repos with at least 2^26=64Mi revisions), we would leave the node tree
half-initialized.

Differential Revision: https://phab.mercurial-scm.org/D4153

mercurial/cext/revlog.c

@@ -1092,15 +1092,15 b' static int nt_delete_node(indexObject *s'
 static int nt_init(indexObject *self)
 {
 	if (self->nt == NULL) {
+		if ((size_t)self->raw_length > INT_MAX / sizeof(nodetreenode)) {
+			PyErr_SetString(PyExc_ValueError, "overflow in nt_init");
+			return -1;
+		}
 		self->nt = PyMem_Malloc(sizeof(nodetree));
 		if (self->nt == NULL) {
 			PyErr_NoMemory();
 			return -1;
 		}
-		if ((size_t)self->raw_length > INT_MAX / sizeof(nodetreenode)) {
-			PyErr_SetString(PyExc_ValueError, "overflow in nt_init");
-			return -1;
-		}
 		self->nt->capacity = self->raw_length < 4
 			? 4 : (int)self->raw_length / 2;