More accurate test in db_fetch_*

Currently, for mysql, mysqli and pgsql, db_fetch_array and db_fetch_object all perform this test:

if ($result) { // perform fetch

It is however less correct than:

if (is_resource($result)) { // perform fetch

Testing for is_resource instead of merely existence can catch more errors, notably by avoiding the occasional error in coding where someone tries to fetch from a query string instead of fetching from the relevant query.

OTOH, if we are ready to let non-resources be passed to the underlying fetch function, as is currently the case, there is not need to test $result and the test can be removed, possibly to be replaced by a try/catch block, although since this depends on PHP5 we probably want to wait for D7 before switching to this model.