pgsql: Avoid core dump after getpwuid_r failure.

Avoid core dump after getpwuid_r failure.

Looking up a nonexistent user ID would lead to a null pointer
dereference.  That's unlikely to happen here, but perhaps
not impossible.

Thinko in commit 4d5111b3f, noticed by Coverity.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2e62fa62d6745ba3bcb0a517d002aff1f3cdefb7

Modified Files
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src/interfaces/libpq/fe-auth.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)