fix(parser): prevent chunk from being garbage collected

[digitcrusher]

Holding a reference to the bytes of a string value which was popped off the stack is not safe since the data is now eligible to be freed by the garbage collector. Although this use-after-free bug is most prominent in a Lua interpreter with multiple threads of execution, it might also happen in a single-threaded one. This hypothesis is backed by section 4.1.3 "Pointers to Strings" of the Lua reference manual:

[…] In general, Lua's garbage collection can free or move memory and then invalidate pointers to strings handled by a Lua state. To allow a safe use of these pointers, the API guarantees that any pointer to a string in a stack index is valid while the string value at that index is not removed from the stack. […] Except for these guarantees, the garbage collector is free to invalidate any pointer to internal strings.