Fix overflow exception when serializing

[25077667]

When the MSB of the initial wchar_t is set, the lowSurrogate would be inferred to be "unsigned".
The if statement would be triggered, and the exception "UTF-16 string has invalid low surrogate" would be thrown.

This needs to be fixed by making the type "signed" and then comparing it to the integer values of the specified constants.

[25077667]

You could also refer to:
The wchar_t type is defined by MIDL as an unsigned short(16-bit) data object [1].

Type wchar_­t is a distinct type that has an implementation-defined signed or unsigned integer type as its underlying type [2].

[1] https://learn.microsoft.com/en-us/windows/win32/midl/wchar-t
[2] https://eel.is/c++draft/basic.types#basic.fundamental-8