fix: add bounds checking to CharStream::get() method

[hawkadrian]

Add bounds checking to prevent undefined behavior when accessing characters beyond the source string.

The method now returns '\0' when attempting to read past the end of input, maintaining consistency with other CharStream methods that handle boundary conditions safely.

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[clonker]

Does this fix an actual issue you have encountered? Otherwise: CharStream behaves in exactly the same way a C/C++ string does. It is already null terminated by virtue of using std::string under the hood, ie, you get '\0' if you access stream.get(str.size()). Anything beyond that index is UB, same as for a regular string in c/c++ as well.