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.

libguile/instructions.h

@@ -31,7 +31,6 @@ enum scm_opcode {
 #include <libguile/vm-i-scheme.i>
 #include <libguile/vm-i-loader.i>
 #undef VM_INSTRUCTION_TO_OPCODE
-  scm_op_last = SCM_VM_NUM_INSTRUCTIONS
 };
 
 SCM_API SCM scm_instruction_list (void);