SegFault attempting to write tag to invalid FLAC file

KID3 will crash with a SegFault when attempting to write a tag to an invalid FLAC file. Tested with the current Git HEAD.

Here is the top of the stack trace.

#0 in FLAC__metadata_iterator_get_block_type () from libFLAC.so.8 #1 in FlacFile::writeTags () #2 in Kid3Application::saveDirectory ()