Add G_SLICE=debug-blocks to test environment
Fix a double free when reading WavPack headers
Fix check for errors when writing WavPack tags
Fix double free in CDDB dialog error path
Use a stack-allocated buffer for WavPack tags
Simplify reading artwork description from Ogg tags
Avoid superfluous use of sizeof(char)
Use the GLib slice allocator where appropriate

When allocating structs, use the GLib slice allocator. Simplify some
allocation code to also zero-initialize the struct.