riched20: fix text replacement with text limit

[Clinton Stimpson]

Here's a resend of the last patch to fix this bug, except with fewer 
code changes.

This patch fixes bug 6985.

Moved the clearing of previous selections so it is done *before* 
calculating the amount of space available to insert new text, or the new 
text might not be inserted in place of the selection.

A test is included.

Thanks,
Clinton Stimpson

ChangeLog:
    Fix insertion of text when near/at the text limit.