SwASCWriter: out of bounds substring access

Regression from 6e0d836ff120ba292ba52f3623a3dd9be04aefc2, when simply
copy-pasting some string to a terminal window, rFltNm is empty.

Change-Id: I874e262ef1a3ebb38d90d9ef4f1b8d3457c5daff
diff --git a/sw/source/filter/ascii/wrtasc.cxx b/sw/source/filter/ascii/wrtasc.cxx
index 87d1002..a7ee4f1 100644
--- a/sw/source/filter/ascii/wrtasc.cxx
+++ b/sw/source/filter/ascii/wrtasc.cxx
@@ -74,7 +74,7 @@ SwASCWriter::SwASCWriter( const OUString& rFltNm )
                break;

    default:
        if( rFltNm.copy( 4 )=="_DLG" )
        if( rFltNm.getLength() >= 4 && rFltNm.copy( 4 )=="_DLG" )
        {
            // use the options
            aNewOpts = GetAsciiOptions();