Avoid improbable null pointer dereference in pgpassfileWarning().

Coverity complained that we might pass a null pointer to strcmp()
if PQresultErrorField were to return NULL.  That shouldn't be possible,
since the server is supposed to always provide some SQLSTATE or other
in an error message.  But we usually defend against such hazards, and
it only takes a little more code to do so here.

There's no good reason to think this is a live bug, so no back-patch.

src/interfaces/libpq/fe-connect.c

@@ -6312,19 +6312,20 @@ passwordFromFile(char *hostname, char *port, char *dbname,
 
 
 /*
- *	If the connection failed, we should mention if
- *	we got the password from the pgpassfile in case that
- *	password is wrong.
+ *	If the connection failed due to bad password, we should mention
+ *	if we got the password from the pgpassfile.
  */
 static void
 pgpassfileWarning(PGconn *conn)
 {
 	/* If it was 'invalid authorization', add pgpassfile mention */
 	/* only works with >= 9.0 servers */
-	if (conn->pgpassfile_used && conn->password_needed && conn->result &&
-		strcmp(PQresultErrorField(conn->result, PG_DIAG_SQLSTATE),
-			   ERRCODE_INVALID_PASSWORD) == 0)
+	if (conn->pgpassfile_used && conn->password_needed && conn->result)
 	{
+		const char *sqlstate = PQresultErrorField(conn->result,
+												  PG_DIAG_SQLSTATE);
+
+		if (sqlstate && strcmp(sqlstate, ERRCODE_INVALID_PASSWORD) == 0)
 			appendPQExpBuffer(&conn->errorMessage,
 					  libpq_gettext("password retrieved from file \"%s\"\n"),
 							  conn->pgpassfile);