[server] Fix isSubthreadMember check in postMessageSend

Summary: Oops! We missed this spot when we introduced -1 as a possible value for the role column of memberships.

Test Plan: log the value on my server (very hard to test notifs locally)

Reviewers: palys-swm

Subscribers: KatPo, zrebcu411, Adrian, atul, subnub

Differential Revision: https://phabricator.ashoat.com/D596