Module doesn't correctly handle _error state

[sjn]

Hei!

When running the test suite with verbose and warnings enabled (see below), a bunch of errors are expressed, that reveal that Net::SNMP->_object_type_validate() isn't called before object destruction. (See the Net::SNMP->DESTROY method).

This is how the warning is reproduced:

$ prove -w -l 
	(in cleanup) FATAL: Invalid blocking object count at /home/sjn/local/lib/perl5/site_perl/5.22.4/Net/SNMP.pm line 2163 during global destruction.
t/01-public-methods.t .... ok
	(in cleanup) FATAL: Invalid blocking object count at /home/sjn/local/lib/perl5/site_perl/5.22.4/Net/SNMP.pm line 2163 during global destruction.
t/02-session.t ........... ok
	(in cleanup) FATAL: Invalid blocking object count at /home/sjn/local/lib/perl5/site_perl/5.22.4/Net/SNMP.pm line 2163 during global destruction.
t/03-close.t ............. ok
	(in cleanup) FATAL: Invalid blocking object count at /home/sjn/local/lib/perl5/site_perl/5.22.4/Net/SNMP.pm line 2163 during global destruction.
t/04-dispatcher.t ........ ok
	(in cleanup) FATAL: Invalid blocking object count at /home/sjn/local/lib/perl5/site_perl/5.22.4/Net/SNMP.pm line 2163 during global destruction.
t/05-get-request.t ....... ok
	(in cleanup) FATAL: Invalid blocking object count at /home/sjn/local/lib/perl5/site_perl/5.22.4/Net/SNMP.pm line 2163 during global destruction.
t/06-get-next-request.t .. ok
	(in cleanup) FATAL: Invalid blocking object count at /home/sjn/local/lib/perl5/site_perl/5.22.4/Net/SNMP.pm line 2163 during global destruction.
t/07-set-request.t ....... ok
	(in cleanup) FATAL: Invalid blocking object count at /home/sjn/local/lib/perl5/site_perl/5.22.4/Net/SNMP.pm line 2163 during global destruction.
t/08-trap.t .............. ok
	(in cleanup) FATAL: Invalid blocking object count at /home/sjn/local/lib/perl5/site_perl/5.22.4/Net/SNMP.pm line 2163 during global destruction.
t/09-get-bulk-request.t .. ok
	(in cleanup) FATAL: Invalid blocking object count at /home/sjn/local/lib/perl5/site_perl/5.22.4/Net/SNMP.pm line 2163 during global destruction.
t/10-inform-request.t .... ok
	(in cleanup) FATAL: Invalid blocking object count at /home/sjn/local/lib/perl5/site_perl/5.22.4/Net/SNMP.pm line 2163 during global destruction.
t/11-snmpv2_trap.t ....... ok
	(in cleanup) FATAL: Invalid blocking object count at /home/sjn/local/lib/perl5/site_perl/5.22.4/Net/SNMP.pm line 2163 during global destruction.
t/12-get-table.t ......... ok
	(in cleanup) FATAL: Invalid blocking object count at /home/sjn/local/lib/perl5/site_perl/5.22.4/Net/SNMP.pm line 2163 during global destruction.
t/13-get-entries.t ....... ok
	(in cleanup) FATAL: Invalid blocking object count at /home/sjn/local/lib/perl5/site_perl/5.22.4/Net/SNMP.pm line 2163 during global destruction.
t/14-misc.t .............. ok
	(in cleanup) FATAL: Invalid blocking object count at /home/sjn/local/lib/perl5/site_perl/5.22.4/Net/SNMP.pm line 2163 during global destruction.
t/15-example.t ........... ok
All tests successful.
Files=15, Tests=237,  0 wallclock secs ( 0.03 usr  0.00 sys +  0.79 cusr  0.09 csys =  0.91 CPU)
Result: PASS