sw: fix warning in SwTextShell::GetState()

warn:sw.core:31398:31398:sw/source/core/bastyp/index.cxx:322: SwIndex::operator--() wraps around

Change-Id: I5c37a6b5e1fe24bc3b6b84e762bad6577583f3d8
Reviewed-on: https://gerrit.libreoffice.org/68127
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>

sw/source/uibase/shells/textsh1.cxx

@@ -1950,7 +1950,7 @@ void SwTextShell::GetState( SfxItemSet &rSet )
                 // Enable it if we have a valid object other than what form shell knows
                 SwPosition aPos(*GetShell().GetCursor()->GetPoint());
                 sw::mark::IFieldmark* pFieldBM = GetShell().getIDocumentMarkAccess()->getFieldmarkFor(aPos);
-                if ( !pFieldBM )
+                if ( !pFieldBM && aPos.nContent.GetIndex() > 0)
                 {
                     --aPos.nContent;
                     pFieldBM = GetShell().getIDocumentMarkAccess()->getFieldmarkFor(aPos);