Corrected IBugtraqProvider2 interface implementation (for TortoiseHg)

Fixes crash in TortoiseHg by correcting TracExplorer's IBugtraqProvider2 interface implementation.

See:

• https://bitbucket.org/tortoisehg/hgtk/issues/1612/index-fault-when-picking-a-ticket
• https://bitbucket.org/tortoisehg/thg/issues/2299/index-fault-when-picking-a-ticket