Fix bug #7518: Assert when selecting in RTL text

Fix bug #7518: Assert when selecting in RTL text

If we are in rtl text, we *need* to check whether the first character is a
space. We can't use the rtl variable for this because this is only
computed on the last row.

This bug was introduced in [0a137e31/lyxgit] to fix the computation for
freespacing paragraphs. We better can just check whether the paragraph is
freespacing or not.