[PATCH 12/19] RISC-V: riscv_set_arch() can fail

[Jiawei]

> In that case neither riscv_rps_as.subset_list nor file_arch_str would be
> set, yet the NULL pointers would be happily passed into functions not
> expecting such.
>
> --- a/gas/config/tc-riscv.c
> +++ b/gas/config/tc-riscv.c
> @@ -345,7 +345,9 @@ riscv_set_arch (const char *s)
>      {
>        as_bad (_("the architecture string of -march and elf architecture "
>          "attributes cannot be empty"));
> -      return;
> +      if (file_arch_str != NULL)
> +    return;
> +      s = DEFAULT_RISCV_ARCH_WITH_EXT;
>      }
>
>    if (riscv_rps_as.subset_list == NULL)
>
LGTM.

So this should still catch cases like a frontend failing to provide the 
archstring, while avoiding a follow-up crash.

Reviewed-by: Jiawei <jiawei@iscas.ac.cn>