SideChain: fixed a rare sync bug

P2Pool could fail to sync if uncles of depth 3 were present at a certain depth in the chain

src/side_chain.cpp

@@ -1368,10 +1368,10 @@ void SideChain::verify(PoolBlock* block)
 	// Deep block
 	//
 	// Blocks in PPLNS window (m_chainWindowSize) require up to m_chainWindowSize earlier blocks to verify
-	// If a block is deeper than m_chainWindowSize * 2 - 1 it can't influence blocks in PPLNS window
+	// If a block is deeper than (m_chainWindowSize - 1) * 2 + UNCLE_BLOCK_DEPTH it can't influence blocks in PPLNS window
 	// Also, having so many blocks on top of this one means it was verified by the network at some point
 	// We skip checks in this case to make pruning possible
-	if (block->m_depth >= m_chainWindowSize * 2) {
+	if (block->m_depth > (m_chainWindowSize - 1) * 2 + UNCLE_BLOCK_DEPTH) {
 		LOGINFO(4, "block " << block->m_sidechainId << " skipped verification");
 		block->m_verified = true;
 		block->m_invalid = false;