A crash fix that was happening in Debug build

This was happening for a specific invalid (non-animated) image in the following scenario:
- frameCount() is called with enough partial data to detect the error. So frameCount() returns 0.
- Next frameBufferAtIndex() is called with the same data. This in turn calls frameCount() --> updateDemuxer(). But as m_haveAlreadyParsedThisData is true, updateDemuxer() returns true; and hence frameCount() returns 1. [Ideally, updateDemuxer() should have returned false].

Added a unit test for the same.

BUG=

Review URL: https://codereview.chromium.org/25345003

git-svn-id: svn://svn.chromium.org/blink/trunk@158606 bbb929c8-8fbe-4397-9dbb-9b2b20218538