When a RelocatingCharacterStream is Seeked, it's buffer_pos_ could be set a non-zero value.
However, UpdateBufferPointers was assuming the position was zero to relocate the buffer_start_
and buffer_end_, which would lead to the stream becoming misaligned. Fix this and add a
unittest and the clusterfuzz script which highlighted the issue.

BUG=chromium:991133

Change-Id: I20dd510b3dcc5df6df058b7e06d2c8a838aef855
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1751782Reviewed-by: Sathya Gunasekaran  <gsathya@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63190}