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Adds two new fuzzers: one to fuzz deserialization of port events in
general, and one to fuzz user message events specifically, which have
an additional layer of parsing beyond the port event header.

A successfully parsed user message event is ultimately how we carry
application payloads end-to-end across message pipes via the public
message pipe API. With these fuzzers in addition to the Channel and
NodeChannel fuzzers, we have fuzz coverage of every part of the stack
between the OS and the generated bindings.

This CL fixes some low-hanging fruit where we (a) weren't properly
handling certain deserialization failure cases, leading to nullptr
deref; and (b) weren't properly rejecting messages with far too
many handles (ostensibly) attached.

Finally this also ensures that Mojo core is initialized in the
other existing fuzzers, since they may also end up deserializing
handles and thus require the global handle table to be set up.

Bug: 897743
Change-Id: Ie5d5f8025728f6e57b2ce46d3c41532bf134eb45
Reviewed-on: https://chromium-review.googlesource.com/c/1352976
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: Oliver Chang <ochang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612043}