Add missing NULL check to FontCache::platformFallbackForCharacter

Add NULL check to FontCache::platformFallbackForCharacter when setting
min-size as data can be NULL if none of the fallback fonts can render
the character.

R=scottmg@chromium.org
BUG=359892

Review URL: https://codereview.chromium.org/225583007

git-svn-id: svn://svn.chromium.org/blink/trunk@170892 bbb929c8-8fbe-4397-9dbb-9b2b20218538