[3.1.2 Backport] Explicit check on xattr length to avoid panics

In https://github.com/couchbase/sync_gateway/blob/master/db/document.go#L519 

body = data[xattrsLen+4:] can panic in some cases where KV sends a malformed xattr content over DCP.

An explicit check on xattr length is needed to avoid this panic

Another related panic can occur on 

pairLen := binary.BigEndian.Uint32(data[pos : pos+4])

can be reproduced with the last test case on TestParseXattr with 

emptyBody, emptyXattr, _, emptyErr := parseXattrStreamData(base.SyncXattrName, "", []byte{math.MaxUint8, math.MaxUint8, math.MaxUint8, math.MaxUint8})

but can also be avoided by checking xattr length before the body assignment. Could use that test to test the panic doesn't occur after adding a check.