hstore: Fix NULL pointer dereference with receive function

hstore: Fix NULL pointer dereference with receive function

The receive function of hstore was not able to handle correctly
duplicate key values when a new duplicate links to a NULL value, where a
pfree() could be attempted on a NULL pointer, crashing due to a pointer
dereference.

This problem would happen for a COPY BINARY, when stacking values like
that:
aa => 5
aa => null

The second key/value pair is discarded and pfree() calls are attempted
on its key and its value, leading to a pointer dereference for the value
part as the value is NULL.  The first key/value pair takes priority when
a duplicate is found.

Per offline report.

Reported-by: "Anemone" <vergissmeinnichtzh@gmail.com>
Reported-by: "A1ex" <alex000young@gmail.com>
Backpatch-through: 14