fix race in which some instruction name symbols could go unmarked

fix race in which some instruction name symbols could go unmarked

* libguile/instructions.c: In loops, replace scm_op_last with
  SCM_VM_NUM_INSTRUCTIONS.
  (fetch_instruction_table): Protect the instruction symbols from
  collection. Before they were only marked by the name->opcode hash
  table, leading to races in which they could be collected.
  (scm_lookup_instruction_by_name): Protect the hash table earlier, as
  it's not actually a stack variable, since it's static.