target/riscv/riscv-qmp-cmds.c: coverity-related fixes

target/riscv/riscv-qmp-cmds.c: coverity-related fixes

Coverity CID 1641401 reports that, in reg_is_ulong_integer(), we're
dereferencing a NULL pointer in "reg1" when using it in strcasecmp()
call. A similar case is reported with CID 1641393.

In theory that will never happen - it's guaranteed that both "reg1" and
"reg2" is non-NULL because we're retrieving them in compile-time from
static arrays. Coverity doesn't know that though.

To make Coverity happier and add a bit more clarity in the code,
g_assert() each token to make it clear that those 2 values aren't
supposed to be NULL ever. Do that in both reg_is_ulong_integer() and
reg_is_u64_fpu().

We're also taking the opportunity to implement suggestions made by Peter
in [1] in both functions:

- use g_strsplit() instead of strtok();
- use g_ascii_strcasecmp() instead of strcasecmp().

[1] https://lore.kernel.org/qemu-devel/CAFEAcA_y4bwd9GANbXnpTy2mv80Vg_jp+A-VkQS5V6f0+BFRAA@mail.gmail.com/

Coverity: CID 1641393, 1641401
Fixes: e06d209aa6 ("target/riscv: implement MonitorDef HMP API")
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20251022125643.588947-1-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>