Fix hasty change about end invariants

`end` is allowed to be <= 0 at the beginning of
checkMaxBetweenExclusive, but not later after checking for the first
range version.

ConflictSet.cpp

@@ -1967,7 +1967,7 @@ bool checkMaxBetweenExclusive(Node *n, int begin, int end,
                               InternalVersionT readVersion) {
   assume(-1 <= begin);
   assume(begin <= 256);
-  assume(0 < end);
+  assume(-1 <= end);
   assume(end <= 256);
   assume(begin < end);