Brain fade in gin_extract_jsonb_path()

I looked into bug #13756,
http://www.postgresql.org/message-id/20151105171933.14035.25039@wrigleys.postgresql.org

The cause of the problem is that gin_extract_jsonb_path() computes
a different hash for the "2" in
{"a":[ ["b",{"x":1}], ["b",{"x":2}]]}
than it does for the "2" in
{"a":[[{"x":2}]]}
And the cause of that is that it supposes that after emitting a hash
for a VALUE, it can just leave stack->hash to be reinitialized by the
next KEY or ELEM. But if the next thing is a sub-object, we propagate
the previous value's hash down into the sub-object, causing values
therein to receive different hashes than they'd have gotten without
the preceding outer-level VALUE.

The attached one-liner fixes it, but of course this means that on-disk
jsonb_path_ops indexes are possibly broken and will need to be reindexed.
I see no way around that ... does anybody else?

regards, tom lane