gp4par: stricter validation for GP_VREF.VREF.
gp4par: stricter validation for GP_VREF.VREF.

This also quashes an unsigned overflow.

azonenberg/openfpga,rqou/openfpga,rqou/openfpga,azonenberg/openfpga,rqou/openfpga,rqou/openfpga,azonenberg/openfpga,azonenberg/openfpga,azonenberg/openfpga,rqou/openfpga