metrics: Add a check for abnormally small messages to prevent crashes

In some situations the |message_size| read from |fd| comes up as 0. In this
case we try to read a negative size for the message body and this leads for
crashes. Add a check to make sure that message_size is at least 4 bytes long
to account for the required 32-bit integer message size field.

BUG=chrome-os-partner:40711
TEST=`FEATURES=test emerge-link metrics`

Change-Id: Ie9adbc8e0e6a9f2c80450bf7ebcb3e05ad1f1f8e
Reviewed-on: https://chromium-review.googlesource.com/276362


Trybot-Ready: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>