Check for NULL in chrome_prefs::UpdatePrefHashStoreIfRequired()

Avoids crash on ChromeOS using multi-profile.

BUG=343261, 346667
TEST=local build, run on desktop Linux
R=gab@chromium.org, sky@chromium.org

Review URL: https://codereview.chromium.org/178703004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253212 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253212