This allows us to either pass a reference, which keeps compatibility
with old code, or to pass a NonnullOwnPtr, which allows us to
comfortably chain streams as usual.
The relevant RFC section from
https://www.rfc-editor.org/rfc/rfc7932#section-9.2
MSKIPBYTES * 8 bits: MSKIPLEN - 1, where MSKIPLEN is
the number of metadata bytes; this field is
only present if MSKIPBYTES is positive;
otherwise, MSKIPLEN is 0 (if MSKIPBYTES is
greater than 1, and the last byte is all
zeros, then the stream should be rejected as
invalid)
So when skip_bytes is zero we need to break and
re-align bytes.
Added the relevant test case that demonstrates this from:
https://github.com/google/brotli/blob/master/tests/testdata/x.compressed
The test-case is heavily inspired by:
https://github.com/google/brotli/blob/master/tests/testdata/x.compressed.01
Or in words: A metadata meta-block containing `Y` (which should be
ignored), and then the actual data (a single `Z`). The bug used to skip
one metadata byte too few, and thus read garbage.
