Fix compiler warning/error: 'rv' may be used uninitialized...

Only if the assert() returns, but okay ...

[ 106s] raft.c: In function 'raft_answer':
[ 106s] raft.c:506:2: error: 'rv' may be used uninitialized in this function [-Werror=maybe-uninitialized]
[ 106s] return rv;
"osc build" has gcc version 4.8.2 20140306