Add expiry check to Beacon::Renewable method

The beacon renewable method originally only checked
that the minimum amount of time has passed since the advertisement
or last renewal. This commit also adds an expiry check, as
an expired beacon cannot be successfully renewed.

src/gridcoin/beacon.cpp

@@ -171,7 +171,7 @@ bool Beacon::Expired(const int64_t now) const
 
 bool Beacon::Renewable(const int64_t now) const
 {
-    return Age(now) > RENEWAL_AGE;
+    return (!Expired(now) && Age(now) > RENEWAL_AGE);
 }
 
 bool Beacon::Renewed() const