PPAPI: Check render_frame_ for validity before use.

BUG=338984

Review URL: https://codereview.chromium.org/145813007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250729 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250729