pdf: Early out from CopyImage if there's nothing to copy.

      If CopyImage is called with src is an empty rectangle (this can happen for
example if one of the resources is not available), then the code ends up trying
to read from null/uninitialized memory. So early out instead in such cases.

BUG=401242
R=thestig@chromium.org

Review URL: https://codereview.chromium.org/465133005

Cr-Commit-Position: refs/heads/master@{#290736}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@290736 0039d316-1c4b-4281-b951-d872f2087c98