Fix crash in set builtin

[caseman]

A command such as set gr would crash in arg handling here:

* thread #18, stop reason = EXC_BAD_ACCESS (code=1, address=0x0)
    frame #0: 0x0000000100019dae tersh`set(state=0x000000010604c600, argc=2, argv=0x0000000105413380, init_args=0x0000000000000000, populated_opts=0x0000700009239a9c) at set.c:197:20
   194              argv_0 = strdup(argv[i++]);
   195              init_args->command_file = argv_0;
   196          } else {
-> 197              argv_0 = strdup(state->frame->argv[0]);
   198          }
   199          argv_free(state->frame->argc, state->frame->argv);
   200          state->frame->argc = argc - i + 1;

An additional check has been added to print the usage text and return an error in this case.