pgsql: In array_position()/array_positions(), beware of empty input arr

In array_position()/array_positions(), beware of empty input array.

These functions incautiously fetched the array's first lower bound
even when the array is zero-dimensional, thus fetching the word
after the allocated array space. While almost always harmless,
with very bad luck this could result in SIGSEGV. Fix by adding
an early exit for empty input.

Per bug #17920 from Alexander Lakhin.

Discussion: https://postgr.es/m/17920-f7c228c627b6d02e%40postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4c40995f61227c579bd7269a829c00013ac66492

Modified Files
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src/backend/utils/adt/array_userfuncs.c | 16 ++++++++++++----
1 file changed, 12 insertions(+), 4 deletions(-)