llmq: Avoid endless loop in GetQuorumRelayMembers (#4796)

* llmq: Avoid endless loop in GetQuorumRelayMembers

regtest quorums can be tiny

* minimize changes, add a note

src/llmq/utils.cpp

@@ -588,6 +588,12 @@ std::set<uint256> CLLMQUtils::GetQuorumRelayMembers(const Consensus::LLMQParams&
     std::set<uint256> result;
 
     auto calcOutbound = [&](size_t i, const uint256& proTxHash) {
+        if (mns.size() == 1) {
+            // No outbound connections are needed when there is one MN only.
+            // Also note that trying to calculate results via the algorithm below
+            // would result in an endless loop.
+            return std::set<uint256>();
+        }
         // Relay to nodes at indexes (i+2^k)%n, where
         //   k: 0..max(1, floor(log2(n-1))-1)
         //   n: size of the quorum/ring