Fix another 64 bit problem in a part select.
Fix another 64 bit problem in a part select.

This patch fixes a problem in the .part/v select when the index is
undefined. The default value needs to be INT_MIN, so when the long
is cast to an int the value is in range.

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