Fix ff_get_guid for short reads or errors.

Current code would just return uninitialized data with no way
to detect this condition.
Instead, fill the whole GUID with 0 in that case.
Fixes valgrind uninitialized data errors in fate-seek-lavf_asf.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
