revlog: remove side effect from failed nt_init()

If nt_init() successfully allocates memory for the node tree but then
runs out of memory while trying to insert nullid into it, it will
leave the node tree pointer set on the index object. That means that
future node tree operations will think that the tree is properly
initialized. It seems very unlikely to make a difference in practice
(if nt_init() runs out of memory, not much else will probably work),
but since I spotted it, I figured I might as well fix it.

Differential Revision: https://phab.mercurial-scm.org/D4028

mercurial/cext/revlog.c

@@ -1112,9 +1112,12 static int nt_init(indexObject *self)
 		self->ntrev = (int)index_length(self) - 1;
 		self->ntlookups = 1;
 		self->ntmisses = 0;
-		if (nt_insert(self, nullid, INT_MAX) == -1)
+		if (nt_insert(self, nullid, INT_MAX) == -1) {
+			free(self->nt);
+			self->nt = NULL;
 			return -1;
 	}
+	}
 	return 0;
 }