Fix nullptr pointer arithmetic in charset decoder

[metsw24-max]

Fix undefined behavior in MbstowcsCharsetDecoder::decode caused by performing pointer arithmetic on a nullptr after a successful mbsrtowcs conversion.

Previously, the code unconditionally computed:

converted = src - cbuf;

However, mbsrtowcs sets src to nullptr after consuming a terminating null character, making the subtraction undefined behavior.

Changes

• Added explicit handling for the src == nullptr case.
• Detect whether the terminating null came from the actual input or the temporary sentinel buffer.
• Preserve normal pointer arithmetic when src is valid.
• Keep existing loop and guard behavior unchanged.

Additional Fix

This also fixes handling of embedded null bytes where the previous logic could incorrectly skip trailing input bytes.