be more like the interpreter in signalling wrong-num-args

be more like the interpreter in signalling wrong-num-args

* libguile/vm-engine.c: Call scm_wrong_num_args in the wrong-num-args
  case, to be more like the interpreter.

* libguile/vm-engine.h (ASSERT): New macro.

* libguile/vm-i-system.c (apply, goto/apply): Assert that nargs >= 2,
  because the compiler should always feed us correct instructions.
  (call/cc): If no values are returned to the continuation, signal
  no_values instead of wrong_num_args.