libpq: change PQconndefaults() to ignore invalid service files

Previously missing or invalid service files returned NULL.  Also fix
pg_upgrade to report "out of memory" for a null return from
PQconndefaults().

Patch by Steve Singer, rewritten by me

src/interfaces/libpq/fe-auth.c

@@ -982,7 +982,7 @@ pg_fe_sendauth(AuthRequest areq, PGconn *conn)
  * if there is an error, return NULL with an error message in errorMessage
  */
 char *
-pg_fe_getauthname(PQExpBuffer errorMessage)
+pg_fe_getauthname(void)
 {
 	const char *name = NULL;
 	char	   *authn;