pgsql: Avoid formally-undefined use of memcpy() inhstoreUniquePairs().

Avoid formally-undefined use of memcpy() in hstoreUniquePairs().

hstoreUniquePairs() often called memcpy with equal source and destination
pointers.  Although this is almost surely harmless in practice, it's
undefined according to the letter of the C standard.  Some versions of
valgrind will complain about it, and some versions of libc as well
(cf. commit ad520ec4a).  Tweak the code to avoid doing that.

Noted by Tomas Vondra.  Back-patch to all supported versions because
of the hazard of libc assertions.

Discussion: https://postgr.es/m/bf84d940-90d4-de91-19dd-612e011007f4@fuzzy.cz

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/47226971e19fa202a42a55118ca2f133bfa6ffbd

Modified Files
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contrib/hstore/hstore_io.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)