Consider the following text as is typically found on README.md

```
[![GitHub Releases Downloads](https://img.shields.io/github/downloads/raysan5/raylib/total)](https://github.com/raysan5/raylib/releases)
```

If the point is say at "r" at "raylib/releases", invoking
(ffap-url-at-point) fails.  this eventually calls
thing-at-point-bounds-of-url-at-point, which has hardcoded behaviour
to, skip over "allowed characters" backwards to find the beginning of
the bound. here it it finds the space character (in "Release
Downloads") and the whole thing fails.

This particular failure can be addressed by supplying the lim
paramater to the skip-chars-backward, as shown in the attached
patch.

does this look like a problem which ought to be solved? and is this
appropriate? (I was going to post on emacs-devel but decided to post
to the bug list instead) -- Best Regards, Madhu