decklink: Fix broken COM string conversion

decklink: Fix broken COM string conversion

WideCharToMultiByte return is the string length without null terminate
character if passed "cchWideChar" does not include the null terminate
character size. Instead of passing the exact string length, pass -1 so that
the API can understand the input string is null terminated already and
returned value from the API includes the character.

Fixes: https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/3023
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5446>