Bug 916850: Fix 'metadata_block_picture' in OGG meta-data parser

When encountering incorrect meta data of type 'metadata_block_picture',
the OGG meta-data parser throws an exception and the Music app does
not list the audio file.

This patch changes the parser to only inspect the values of the meta-
data fields that it is interested in. All other values will be skipped.

Signed-off-by: Thomas Zimmermann <tdz@users.sourceforge.net>