pgsql: Handle PQresultErrorField(PG_DIAG_SQLSTATE) returning NULL in st

Handle PQresultErrorField(PG_DIAG_SQLSTATE) returning NULL in streamutil.c.

In ff27db5d I missed that PQresultErrorField() may return NULL if
there's no sqlstate associated with an error.

Spotted-By: Coverity
Reported-By: Michael Paquier
Discussion: CAB7nPqQ3o10SY6NVdU4pjq85GQTN5tbbkq2gnNUh2fBNU3rKyQ@mail.gmail.com
Backpatch: 9.5, like ff27db5d

Branch
------
REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/2e6f6f3abe6fd249cc8a4d5eb194295ac3988b19

Modified Files
--------------
src/bin/pg_basebackup/streamutil.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

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