Check image extraction for activity stream featured image

[DJPaul]

Per ​https://buddypress.org/support/topic/featured-image-wont-show-up-on-member-activity-feed/#post-258998

While responding to a support request, I looked at the code, and I think there's a gotcha in the logic that extracts media from a block of text. I think the case of having more than 3 paragraphs (in the source text) and no images will result in no media showing, even if other media exists.