raft: fix spurious split-vote

Due to a typo raft candidate counted a vote for another node as a vote
for self in its split-vote detector. This could lead to spurious
split-vote detection in cases when another node wins elections with a bare
minimum of votes for it (exactly a quorum of votes).

Closes #8698

NO_DOC=bugfix

(cherry picked from commit 2afde5b1)