fix(readPostingList): Return error instead of panic (dgraph-io#5877)

We've seen some panics like "proto: illegal wireType 6" on trying to read the
posting lists. We haven't found the fix yet. The issue could be because badger
is returning incorrect values for the posting list. Since we haven't been able
to reproduce it, this is a stop-gap solution to return an error instead of
crashing dgraph.

This is not the fix, this will return an error instead of
causing a panic.

Fixes DGRAPH-1778

posting/mvcc.go

@@ -317,7 +317,9 @@ func ReadPostingList(key []byte, it *badger.Iterator) (*List, error) {
 		case BitDeltaPosting:
 			err := item.Value(func(val []byte) error {
 				pl := &pb.PostingList{}
-				x.Check(pl.Unmarshal(val))
+				if err := pl.Unmarshal(val); err != nil {
+					return err
+				}
 				pl.CommitTs = item.Version()
 				for _, mpost := range pl.Postings {
 					// commitTs, startTs are meant to be only in memory, not