osd: read into correct variable for magic string

4d140a71a1a48081b449b7d8dde808eb6e74c6b2 refactored this and introduced a
bug. peek_meta() was accidentally reading into magic, then replacing magic
with val, which was always the empty string, resulting in the osd always
failing to start due to 'mismatched' magic values.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>