Pepper: Tiny fix for X509CertificateParseDER.

I came across this while browsing around through Pepper code. The existing code
is unsafe if der.size() == 0. This change prevents us from making an
out-of-bounds access.

BUG=

Review URL: https://codereview.chromium.org/32823021

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230758 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230758