FLACExtractor: Don't check size of first STREAMINFO

The size of the first STREAMINFO is not necessarily 042, as some
encoders may fill it with padding. The metadata parser handles
premature EOF just fine, so we don't need to be strict about it.

Change-Id: Ic077633c2622bb54402cdd8f04d30cd2b7776727

Patchset: 1
http://review.cyanogenmod.org/#/c/55025/