tdf#158837 hyphenation crash in: int rtl::str::indexOfStr_WithLength

the code was already dodgy, passing in an invalid nPos, which triggers
an assert.

However, it appears that previously that dodginess was ignored, until
    commit 5ebccaa075
    author	Noel Grandin 	Sat Apr 10 17:27:08 2021 +0200
    use std lib for O[U]String indexOfStr
which turned it into a crash.

Add a check to avoid the crash, no idea what the right fix would look
like.

Change-Id: I826b9af0b2d9c7c297214ce2d54c664fb573edd2
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/161271
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

cui/source/dialogs/hyphen.cxx

@@ -171,6 +171,8 @@ OUString SvxHyphenWordDialog::EraseUnusableHyphens_Impl()
                 aLeft = aLeft.replaceFirst( aTmp, "", &nPos );
                 if (nPos != -1)
                     ++m_nHyphenationPositionsOffset;
+                if (nPos >= aLeft.getLength()) // tdf#158837
+                    break;
             }
             aTxt = aTxt.replaceAt( 0, nPos2, aLeft );
         }