Revision 6551

(Db_traverse_tag_find) fix a rare segfault

found accidently

could be triggered by the following (calling enough times either
requires() or provides() while traversing rpm db) which resulting in
an "Invalid read of size 8":

use URPM;
my $db = URPM::DB::open()->traverse_tag_find('whatprovides', 'kernel',
sub {
    my ($p) = @_;
    warn $p->requires, $p->requires, $p->requires, $p->requires, $p->requires,
    $p->requires, $p->requires, $p->requires, $p->requires, $p->requires, $p->requires,
    $p->requires, $p->requires, $p->requires, $p->requires, $p->requires, $p->requires;
    return 1;
});

bug introduced in commit r4947 on June 14 2012:
"(_run_cb_while_traversing) introduce this helper"