Fix crash on invalid date object

[bnoordhuis]

mini_racer converts JS objects to Ruby objects. Ruby's C API functions call rb_raise() on invalid inputs, and rb_raise() in turn is a glorified longjmp() that jumps up the stack.

Longjmp and C++ destructors do not play well together. It left V8 in an undefined state, resulting in random "Disposing the isolate that is entered by a thread" fatal errors.

Fixes: #316

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Easiest to review with ?w=1 (link), the bulk is whitespace change because of an extra level of indent.

The title says "invalid date object" but this pull request in fact addresses a wider range of bugs around value conversion.

There is at least one other instance around JSON parsing. I'll open another PR for that.

[bnoordhuis]

I wonder if I found a truffleruby bug...

MiniRacerTest#test_date_nan:
NoMethodError: undefined method `source_location' for "<internal:core> core/float.rb:114:in `to_i'":Thread::Backtrace::Location
    lib/mini_racer/truffleruby.rb:199:in `block in translate'
    lib/mini_racer/truffleruby.rb:199:in `select'
    lib/mini_racer/truffleruby.rb:199:in `translate'
    lib/mini_racer/truffleruby.rb:132:in `eval_unsafe'
    lib/mini_racer.rb:237:in `block (2 levels) in eval'
    lib/mini_racer.rb:358:in `timeout'
    lib/mini_racer.rb:236:in `block in eval'
    lib/mini_racer.rb:234:in `synchronize'
    lib/mini_racer.rb:234:in `eval'
    test/mini_racer_test.rb:257:in `test_date_nan'