Fix assertion when checking if change in selection

Fix assertion when checking if change in selection

The check for the iterator being in the same paragraph as the end of
selection was incorrect, because paragraphs in different cells could
have the same pit. We now additionally condition on having the same
idx.

This commit amends d1279875 (and thus 23de5e5e).

For discussion, see:

  https://www.mail-archive.com/search?l=mid&q=a5afd0c01a0eb9a84fd4d050d15eb23016d6f38a.camel%40lyx.org

Thanks to Jürgen.