Re: RISC-V: riscv_parse_subset_t's isa_spec is only parser input

check_compiler_available fails with a ld segfault linking
int main (void) { return 0; }

Program received signal SIGSEGV, Segmentation fault.
0x0000000000466a64 in riscv_parse_add_subset (rps=rps@entry=0x7fffffffd450, subset=subset@entry=0x962ca0 "zaamo", major=-1, minor=-1, implicit=implicit@entry=true) at /home/alan/src/binutils-gdb/bfd/elfxx-riscv.c:1947
1947	    riscv_get_default_ext_version (*rps->isa_spec, subset,
(gdb) p rps->isa_spec
$1 = (const enum riscv_spec_class *) 0x0

This was triggered when calling riscv_merge_attributes for crti.o
which had
(gdb) p ibfd->tdata.elf_obj_data->known_obj_attributes[OBJ_ATTR_PROC][Tag_RISCV_arch]
$5 = {type = 2, i = 0, s = 0x97e4c0 "rv64i2p1_m2p0_a2p1_f2p2_d2p2_c2p0_zicsr2p0_zifencei2p0_zmmul1p0"}

Fix the segfault by reinstating a NULL check removed with commit
413cf36d8e81