pgsql: Fix corner-case coredump in _SPI_error_callback().

Fix corner-case coredump in _SPI_error_callback().

I noticed that _SPI_execute_plan initially sets spierrcontext.arg = NULL,
and only fills it in some time later. If an error were to happen in
between, _SPI_error_callback would try to dereference the null pointer.
This is unlikely --- there's not much between those points except
push-snapshot calls --- but it's clearly not impossible. Tweak the
callback to do nothing if the pointer isn't set yet.

It's been like this for awhile, so back-patch to all supported branches.

Branch
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REL9_4_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/239b01e313b5c3bc90d383b0a345e0549fe9ba96

Modified Files
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src/backend/executor/spi.c | 3 +++
1 file changed, 3 insertions(+)