lib/Cxn.pm: Silence a warning (in 5.14)

I think this warning actually happens everywhere, but because of
how it happens, it gets masked by older versions of Perl.

Basically, Cxn's DESTROY method checks if $self->{dbh}, and if true,
calls ->disconnect on that. However, during global destruction --
when DESTROY is called -- you have no assurances that DESTROY
will be called before the dbh was already reaped.

This commit makes sure that the $self->{dbh} we have inside DESTROY
is actually a reference.

lib/Cxn.pm

@@ -193,7 +193,7 @@ sub name {
 
 sub DESTROY {
    my ($self) = @_;
-   if ( $self->{dbh} ) {
+   if ( $self->{dbh} && ref($self->{dbh}) ) {
       PTDEBUG && _d('Disconnecting dbh', $self->{dbh}, $self->{name});
       $self->{dbh}->disconnect();
    }