stream: actually drop unneeded buffer

The buffer can be larger than the normal size when "peeking" is used
(such as done with some file formats, where a large number of bytes masy
need to be "peeked" at the beginning, because FFmpeg). Once normal
operation resumes, it's supposed to free this buffer again. Apparently
this didn't happen as intended, because normal reading did have no way
to discard back buffer before/while resizing the buffer. There's only a
path for discarding the back buffer when actually reading.

It seems like this unfortunately needs 2 code paths for discarding old
data. Just put it into stream_resize_buffer(), where it's rather
non-tricky (discarding can be done by adjusting the copy offset when
moving data to the new allocation). The function now drops old data if
it doesn't fit into the allocation. The caller must ensure that the new
size is sufficient; the function signature changes only so the size of
the implicitly guaranteed kept part can be checked with assert().

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