Somehow, accessible text can lose focus and so when
attempting to set the cursor, the string we were just
working with may no longer be available, so verify that
the cursor position is still valid before setting it.

If the text can just disappear like that, then potentially
we are still forcing the cursor into the wrong place
(since the string is different than expected).  However
we can at least make sure the program doesn't
crash/exit without saving...

Change-Id: I8dcaf4625d187cb75008bdc0af0e3a546090a018
Reviewed-on: https://gerrit.libreoffice.org/19414Reviewed-by: Justin Luth <justin_luth@sil.org>
Tested-by: Justin Luth <justin_luth@sil.org>
(cherry picked from commit d9cf4744)
Reviewed-on: https://gerrit.libreoffice.org/19418Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>
(cherry picked from commit 9f008e2a)