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

src/bin/pg_basebackup/streamutil.c

@@ -340,7 +340,9 @@ CreateReplicationSlot(PGconn *conn, const char *slot_name, const char *plugin,
 	{
 		const char *sqlstate = PQresultErrorField(res, PG_DIAG_SQLSTATE);
 
-		if (slot_exists_ok && strcmp(sqlstate, ERRCODE_DUPLICATE_OBJECT) == 0)
+		if (slot_exists_ok &&
+			sqlstate &&
+			strcmp(sqlstate, ERRCODE_DUPLICATE_OBJECT) == 0)
 		{
 			destroyPQExpBuffer(query);
 			PQclear(res);