pgsql: Tighten coding for non-composite case in plperl's return_next.

Tighten coding for non-composite case in plperl's return_next.

Coverity complained about this code's practice of using scalar variables
as single-element arrays. While that's really just nitpicking, it probably
is more readable to declare them as arrays, so let's do that. A more
important point is that the code was just blithely assuming that the
result tupledesc has exactly one column; if it doesn't, we'd likely get
a crash of some sort in tuplestore_putvalues. Since the tupledesc is
manufactured outside of plperl, that seems like an uncomfortably long
chain of assumptions. We can nail it down at little cost with a sanity
check earlier in the function.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b4cc35fbb709bd6fcae8998f041fd731c9acbf42

Modified Files
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src/pl/plperl/plperl.c | 28 +++++++++++++++++-----------
1 file changed, 17 insertions(+), 11 deletions(-)