test/fuzz: fixed ElseIfBlock serialization

Changed `else if` to `elseif`. There was a typo in the
serializer that led to unclosed `if` statements.

On a set of 50000 protobuf struct samples generates 863
(3% of all errors) fewer errors.

NO_CHANGELOG=internal
NO_DOC=fuzzer fix

(cherry picked from commit 8c1e75e7)