bp_activity_thumbnail_content_images() assumes img height/width is always set

[DJPaul]

bp_activity_thumbnail_content_images parses <img> tags in the main content of an activity object, when generating a summary for the activity stream.

The current implementation assumes that the img tag will always contain width= and height= properties. If the content doesn't contain these properties inside img tags, it causes PHP Warnings.

I discovered this while working on another feature patch, so I don't have an easy unit test ready to prove the bug, though I've attached a patch.

[djpaul]

In 9456:

Activity: update bp_activity_thumbnail_content_images to check that height/width attributes exist on <img> tags before trying to reference them.

Prevents PHP Notices in edge cases involving developers putting custom content through this function.

Fixes #6159