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