exo: Fix a nullptr dereference

In some cases (e.g. the linked bug) source can be null. SetSelection is
already a no-op for nullptr source (and invalid for nullptr data_device)
so there's no change in behaviour for the case where event_type !=
nullopt, and it fixes a nullptr dereference when event_type == nullopt.

Bug: 1138761
Test: Deploy, launch evolution, Chrome doesn't crash
Change-Id: I23da80e418834f5c1e38eecd2f0b140638b63a8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2487022


Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Daichi Hirono <hirono@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Fergus Dall <sidereal@google.com>
Commit-Queue: David Munro <davidmunro@google.com>
Cr-Commit-Position: refs/heads/master@{#820056}