target/riscv: Use g_assert() for the predicate() NULL check

target/riscv: Use g_assert() for the predicate() NULL check

At present riscv_csrrw_check() checks the CSR predicate() against
NULL and throws RISCV_EXCP_ILLEGAL_INST if it is NULL. But this is
a pure software check, and has nothing to do with the emulation of
the hardware behavior, thus it is inappropriate to return illegal
instruction exception when software forgets to install the hook.

Change to use g_assert() instead.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Weiwei Li<liweiwei@iscas.ac.cn>
Message-ID: <20230228104035.1879882-4-bmeng@tinylab.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>