disas/riscv: Guard dec->cfg dereference for host disassemble

disas/riscv: Guard dec->cfg dereference for host disassemble

For riscv host, it will set dec->cfg to zero. Thus we shuld guard
the dec->cfg deference for riscv host disassemble.

And in general, we should only use dec->cfg for target in three cases:

1) For not incompatible encodings, such as zcmp/zcmt/zfinx.
2) For maybe-ops encodings, they are better to be disassembled to
   the "real" extensions, such as zicfiss. The guard of dec->zimop
   and dec->zcmop is for comment and avoid check for every extension
   that encoded in maybe-ops area.
3) For custom encodings, we have to use dec->cfg to disassemble
   custom encodings using the same encoding area.

Signed-off-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20241206032411.52528-1-zhiwei_liu@linux.alibaba.com>