pgsql: plperl: Correctly handle empty arrays in plperl_ref_from_pg_arra

plperl: Correctly handle empty arrays in plperl_ref_from_pg_array.

plperl_ref_from_pg_array() didn't consider the case that postgrs arrays
can have 0 dimensions (when they're empty) and accessed the first
dimension without a check. Fix that by special casing the empty array
case.

Author: Alex Hunsaker
Reported-By: Andres Freund / valgrind / buildfarm animal skink
Discussion: 20160308063240.usnzg6bsbjrne667@alap3.anarazel.de
Backpatch: 9.1-

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/6041d388c40a23ee0e7b313baeceb8998eed54bf

Modified Files
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src/pl/plperl/plperl.c | 29 ++++++++++++++++++++---------
1 file changed, 20 insertions(+), 9 deletions(-)