Tracer: Decode an all-zero instruction as c.unimp
Tracer: Decode an all-zero instruction as c.unimp

Fixes #396

lowRISC/ibex,AmbiML/ibex,AmbiML/ibex,AmbiML/ibex,AmbiML/ibex,lowRISC/ibex,lowRISC/ibex,lowRISC/ibex