protect against short file

Change-Id: I20c6a6fb34485f42c2a828618e8918d8a441860a
Reviewed-on: https://gerrit.libreoffice.org/42389Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

sax/source/expatwrap/xml2utf.cxx

@@ -221,8 +221,7 @@ bool XMLFile2UTFConverter::scanForEncoding( Sequence< sal_Int8 > &seq )
     }
 
     // first level : detect possible file formats
-    if( ! strncmp( reinterpret_cast<const char *>(pSource), "<?xml", 5 ) ) {
-
+    if (seq.getLength() >= 5 && !strncmp(reinterpret_cast<const char *>(pSource), "<?xml", 5)) {
         // scan for encoding
         OString str( reinterpret_cast<const char *>(pSource), seq.getLength() );