Added early return in WebCore::TextResourceDecoder::checkForCSSCharset to fix buffer overflow

Backported from Webkit: http://trac.webkit.org/changeset/169318 (patch by ddkilzer@apple.com) with a modified test to work for Blink.

BUG=376800
R=eseidel

Review URL: https://codereview.chromium.org/298023006

git-svn-id: svn://svn.chromium.org/blink/trunk@174925 bbb929c8-8fbe-4397-9dbb-9b2b20218538