Before this commit, the passed in length would always be given to the RtlCaptureStackBackTrace call. Now we always give the length of the actual buffer we're using (the addr_buf_stack size of 32 or the passed in length if it's larger than 32; this matches what the doc comment says the function was meant to be doing as well).

This was causing empty stack traces for things like the GeneralPurposeAllocator leak checking.

Fixes #6687