The _vpiNexusId property must return zero when undefined
The _vpiNexusId property must return zero when undefined

This was created by a user contributed patch. It resulted in all objects
that cannot have aliases being aliased.

CastMi/iverilog,themperek/iverilog,themperek/iverilog,CastMi/iverilog,themperek/iverilog,CastMi/iverilog