Release note update
Fix a potential 64-bit pointer arithmetic issue
Treat negative lexer input_offset as an internal error

Before the change a negative input offset would lead to memory unsafe
behavior.  This is correct as such because a negative input offset
should never happen (so an assert is sufficient).  However, the compiler
saves and restores lexer "points" to and from duk_tval numbers, so if
duk_tval representation is chosen improperly or something similar happens,
the input offset could be corrupted (again, this is only the case if
something is pretty badly wrong anyway).

The new behavior is to throw an internal error for negative input offsets.
This is explicit and memory safe, and protects against potential compiler
bugs too.
Portability note for VS2010/X64