Check solution length in validate challenge

Found by Fuzz Bot.

applications/proof_of_work_app.go

@@ -87,6 +87,9 @@ func (powapp *ProofOfWorkApplication) solveChallenge(challenge []byte, prng core
 
 // ValidateChallenge returns true if the message and spamguard pass the challenge
 func (powapp *ProofOfWorkApplication) validateChallenge(challenge []byte, solution []byte) bool {
+	if len(solution) != 32 {
+		return false
+	}
 	solve := make([]byte, len(challenge)+32)
 	copy(solve[0:], solution[0:32])
 	copy(solve[32:], challenge[:])