[USF] return if calloc fails.

Close CID: 171

modules/codec/subtitles/subsusf.c

@@ -643,7 +643,7 @@ static void ParseUSFHeaderTags( decoder_t *p_dec, xml_reader_t *p_xml_reader )
                     if( ! p_style )
                     {
                         free( psz_node );
-                        break;
+                        return VLC_ENOMEM;
                     }
                     /* All styles are supposed to default to Default, and then
                      * one or more settings are over-ridden.