revlog: cache delta base value under -1

Such base are invalid so we better report them early.

mercurial/cext/revlog.c

@@ -863,6 +863,13 b' static inline int index_baserev(indexObj'
 		    rev, result);
 		return -2;
 	}
+	if (result < -1) {
+		PyErr_Format(
+		    PyExc_ValueError,
+		    "corrupted revlog, revision base out of range: %d, %d",
+		    rev, result);
+		return -2;
+	}
 	return result;
 }