fix: close fd on mmap failure in CreateShmBuffer

[MrMartyK]

The ftruncate error path closes the fd, but the mmap error path doesn't. This leaks the fd (and the backing tmpfs pages, since the file is already unlinked).

[oSoMoN]

The fd can be closed regardless of the return value of mmap. From the man page:

After the mmap() call has returned, the file descriptor, fd, can be closed immediately without invalidating the mapping.

Note that there is one other place in the code base where a file descriptor isn't closed after being successfully mmapped (in stream_handle_add_buffer() in src/pipewire.cpp).