pgsql: Fix thinkos in LookupFuncName() for function name lookups

Fix thinkos in LookupFuncName() for function name lookups

This could trigger valgrind failures when doing ambiguous function name
lookups when no arguments are provided by the caller.  The problem has
been introduced in aefeb68, so backpatch to v10.  HEAD is fine thanks to
the refactoring done in bfb456c1.

Reported-by: Alexander Lakhin
Author: Alexander Lakhin, Michael Paquier
Discussion: https://postgr.es/m/3d068be5-f617-a5ee-99f6-458a407bfd65@gmail.com
Backpatch-through: 10

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/52cdbdc42c23e4f9ae1e62e95bd85d11c030d3f6

Modified Files
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src/backend/parser/parse_func.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)