Fix NULL pointer access error in SynthDriverAudioStream (#17607)
This is a fix for the NULL pointer access error introduced by #17592 and reported in this comment.
According to Microsoft's documentation, the pcbWritten parameter in ISequentialStream::Write and the plibNewPosition parameter in IStream::Seek can be NULL, in which case the function should ignore the output parameter and succeed.
Description of user facing changes
None
Description of development approach
ISequentialStream_RemoteWrite and IStream_RemoteSeek are changed to use the low level implementation. This makes checking the output parameter easier. Then, check if the output pointer is NULL before assigning the output value.
