read_wide_string hides errors if the string is nil

Puppet Version: 5.5.2 (PE 2018.1.2 Master & Agent)
Puppet Server Version: N/A
OS Name/Version: Windows Server 2012r2

As part of https://puppetlabs.zendesk.com/agent/tickets/32364 it was found that there was a bug in the error handler in the read_wide_string method (https://github.com/puppetlabs/puppet/blob/master/lib/puppet/util/windows/api_types.rb#L56-L63)

Where if str is nil then an error is raised during the debug statement and raises the wrong error. Instead the Puppet.debug statement should never error and raise the real error