Propagate failure of MetadataBuilder creation in CreateUpdateSnapshots

In a device with malformed metadata, the MetadataBuilder returns a
nullptr that may cause segmentation faults when using the builder.
Fix by handling the nullptr exception in CreateUpdateSnapshot and
propagating the error to its caller.

Bug: n/a
Test: libsnapshot_test
Change-Id: Ie9148a552cf4bb223ab8d54b1d30d2b13d92bb22
Signed-off-by: Alessio Balsini <balsini@google.com>