sw: fix insertion of hyperlink inside hyperlink...

... when either the start or the end position of the new hint is equal
to an existing hint.  There is not just 1 case here but 3 different
ones; don't attempt to insert hints with start > end.

Change-Id: I39cf8a352f67d77b56b0d01de5872f4d341f6bdd
fdo#85562: sw: fix crash on insert of input field and set variable field

When a new field is inserted it doesn't have a SwFmtFld yet and it is
also not yet in the document so it is not necessary to listen for its
deletion.

(regression from bbd97fe57d9ec184ef6aee36bd57d6d7e53b4719)

Change-Id: I23c18aa8c490a6ad1de63a33f87679cc2358d7f5