libsnapshot: Don't PrepMergeOps on resume

If we're reading up to a label, we're resuming setting up the file, and
there is no reason to expect the ops we require for sequence ops to be
present. In that case, skip prepping for merge, and return an empty
merge iterator if it is mistakenly requested.

Test: cow_api_test CowTest.ResumeSeqOp
Change-Id: Idd93bd4c4209197b9728fcb21a7191aae971b62d