vcl: Assert that a non-negative nXPos is always found

Change-Id: Ifb85cac1289af94102947b9064a8c12ca65ad43b
Reviewed-on: https://gerrit.libreoffice.org/19404Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/source/outdev/text.cxx

@@ -1124,6 +1124,7 @@ bool OutputDevice::GetCaretPositions( const OUString& rStr, long* pCaretXArray,
     for( i = 0; i < 2 * nLen; ++i )
         if( pCaretXArray[ i ] >= 0 )
             break;
+    assert(i < 2 * nLen); // otherwise i may go beyond the end of pCaretXArray
     long nXPos = pCaretXArray[ i ];
     for( i = 0; i < 2 * nLen; ++i )
     {