Fix undefined behavior in rpc/server.cpp

Summary: Calling std::move() on command while dereferencing GetName() is undefined.

Test Plan: ninja check

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: deadalnix, Fabien, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2632