0x00000000 incorrectly decoded as c.addi4spn

[ripopov]

Encodings with bits [15:0] all zeros are defined as illegal instructions. These instructions are considered to be of minimal length: 16 bits if any 16-bit instruction-set extension is present, otherwise 32 bits. The encoding with bits [ILEN-1:0] all ones is also illegal; this instruction is considered to be ILEN bits long.

https://riscv.github.io/riscv-isa-manual/snapshot/unprivileged/#%5C_base_instruction_length_encoding

Issue discovered in Surfer waveform viewer, that uses instruction-decoder to translate signals to RISCV instructions:
https://gitlab.com/surfer-project/surfer/-/work_items/553

[FRoith]

yes, that is true (see #22 ), I just have not yet gotten around to adding the feature to "filter out" illegal variable assignments, to "un-match" an instruction if it is a reserved code point