Fix segfault and build error on Linux

The removal of setjmp/longjmp in #22 has introduced a segfault on Linux platforms, as shown on Travis CI.

These crashes have been caused by missing early returns after `ParseStringToStream`.  Additionally, an early return has been added after the `ParseArray/ParseObject` in the top-level code by jumping to the cleanup part directly.

Last, but not least, constness has been casted away in my CopyFrom function (see #22).