Imlement vpi_handle(vpiScope, 0) for NULL handles
Imlement vpi_handle(vpiScope, 0) for NULL handles

This is not part of the standard, but it convenient, and may be
used by the libveriuser to implement some bits of PLI1.

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