pgsql: Check for BuildIndexValueDescription returning NULL in gist_page

Check for BuildIndexValueDescription returning NULL in gist_page_items

Per Coverity. BuildIndexValueDescription() cannot actually return NULL in
this instance, because it only returns NULL if the user doesn't have the
required privileges, and this function can only be used by superuser. But
better safe than sorry.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5d1e5c8b758770186b005a1c3888b05e37af79c5

Modified Files
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contrib/pageinspect/gistfuncs.c | 12 +++++++++---
1 file changed, 9 insertions(+), 3 deletions(-)