Fix seeking to anything except 0 with BufferedStream.
Fix seeking to anything except 0 with BufferedStream.

Previously, given a non-zero location to seek to, we would always
seek to the start of the second buffered chunk.

mindw/html5lib-python,mindw/html5lib-python,gsnedders/html5lib-python,html5lib/html5lib-python,dstufft/html5lib-python,html5lib/html5lib-python,dstufft/html5lib-python,alex/html5lib-python,mgilson/html5lib-python,ordbogen/html5lib-python,mgilson/html5lib-python,dstufft/html5lib-python,html5lib/html5lib-python,mgilson/html5lib-python,ordbogen/html5lib-python,gsnedders/html5lib-python,alex/html5lib-python,alex/html5lib-python,mindw/html5lib-python,ordbogen/html5lib-python