Bug 947918: Support seeking to end of BlobView buffer

Reading or advancing through all bytes in a BlobView buffer sets
the index to the length of the buffer. This patch changes 'seek'
to work the same way, so it's possible to seek to the buffer's
end.

Signed-off-by: Thomas Zimmermann <tdz@users.sourceforge.net>
Merge pull request #14502 from tdz/bug-947918

Bug 947918: Support seeking to end of BlobView buffer r=padenot