Fix a crash in SerialIoHandler.

Fix a crash in SerialIoHandler.

When a receive error occurs, the SerialConnection pauses receives, which
triggers a call to CancelRead(). This should be a no-op as this occurs
after the previous read completed with an error, but
pending_read_buffer_ isn't cleared until after the error is reported to
SerialConnection. Thus, on posix, a call to ReadDone is enqueued, but
the pending read is cleared before it runs, leading to a crash. This
change clears pending_read_buffer_ and pending_write_buffer_ before
calling Done() or DoneWithError().

BUG=410331
Committed: https://crrev.com/f4c09a1101b323c71862728bc86e882d67c1404b
Cr-Commit-Position: refs/heads/master@{#293684}

[Sam McNally, 2014-09-08 01:20:23]

sammc@chromium.org changed reviewers:
+ rockot@chromium.org

[Ken Rockot(use gerrit already), 2014-09-08 06:14:35]

Ah, thanks for the explanation. lgtm

[Sam McNally, 2014-09-08 06:19:36]

The CQ bit was checked by sammc@chromium.org

[commit-bot: I haz the power, 2014-09-08 06:20:20]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/sammc@chromium.org/551713002/1

[commit-bot: I haz the power, 2014-09-08 07:14:41]

Committed patchset #1 (id:1) as 3de8b570ea452a003c012f379e6f58de3aa8e422

[commit-bot: I haz the power, 2014-09-10 03:44:56]

Patchset 1 (id:??) landed as
https://crrev.com/f4c09a1101b323c71862728bc86e882d67c1404b
Cr-Commit-Position: refs/heads/master@{#293684}