revlog: handle errors from index_node() in nt_insert() and index_slice_del()

Same idea as in a9d9802d577e (revlog: don't say "not found" on
internal error, 2018-05-04).

Differential Revision: https://phab.mercurial-scm.org/D3558

mercurial/cext/revlog.c

@@ -1068,10 +1068,12 static int nt_insert(indexObject *self,
 			return 0;
 		}
 		if (v < 0) {
-			const char *oldnode = index_node(self, -(v + 1));
+			const char *oldnode = index_node_existing(self, -(v + 1));
 			int noff;
 
-			if (!oldnode || !memcmp(oldnode, node, 20)) {
+			if (oldnode == NULL)
+				return -1;
+			if (!memcmp(oldnode, node, 20)) {
 				n->children[k] = -rev - 1;
 				return 0;
 			}
@@ -1850,9 +1852,10 static int index_slice_del(indexObject *
 			Py_ssize_t i;
 
 			for (i = start + 1; i < self->length - 1; i++) {
-				const char *node = index_node(self, i);
+				const char *node = index_node_existing(self, i);
+				if (node == NULL)
+					return -1;
 
-				if (node)
 					nt_insert(self, node, -1);
 			}
 			if (self->added)