Fix signed/unsigned warnings
Fix signed/unsigned warnings

Fixed some signed/unsigned warnings, including one that caused an
actual runtime error if not dealt with safely.

Signed-off-by: Stephen Williams <9ce5770b3bb4b2a1d59be2d97e34379cd192299f@icarus.com>
CastMi/iverilog,themperek/iverilog,themperek/iverilog,themperek/iverilog,CastMi/iverilog,CastMi/iverilog