Media Extractor: avoid notices when preg_match fails

More details here:
http://wordpress.org/support/topic/non-robust-code-in-jetpack_media_meta_extractorget_images_from_html
Media Extractor: pass all variables to isset()

See #46
Merge pull request #46 from jeherve/1743114-notices-media-extractor

Media Extractor: avoid notices when preg_match fails