libstagefright: Fixing a crash in EOS for thumbnail generation.

Initializing an uninitialized variable and fixing
a crash. At EOS OMXCodec::read was returning OK and
a NULL buffer. This is not a valid return combination
as client expects buffer to be non-NULL when there
is no error. With this change OMXCodec::read
will return ERROR_END_OF_STREAM with a NULL buffer
which is a valid return combination. This change
is specific to hardware thumbnail generation.

Change-Id: I83ebdc69e2195b903ea652cb46bf91df48457eec