Fix eat-heads guard: skip when decorated_heads is empty (p2pool match)

p2pool node.py:359 guards eat-heads with 'if decorated_heads:' —
when the verified chain has no scored heads, the entire head-eating
loop is skipped. c2pool was missing this guard, causing all heads
older than 300s to be eaten when top5 was empty (e.g. after chain
disruption). This collapsed 34 heads to 2, fragmenting the chain
from depth 1072 to 49 with 72 disconnected tails.

Author: frstrtr

src/impl/ltc/node.cpp

@@ -1381,6 +1381,7 @@ void NodeImpl::clean_tracker()
     //   1. Top-5 scored heads (decorated_heads[-5:])
     //   2. Heads seen < 300s ago
     //   3. Unverified heads whose tail has recent child activity (< 120s)
+    if (!m_last_top5_heads.empty())  // p2pool node.py:359: if decorated_heads:
     {
         // Build top-5 set for O(1) lookup
         std::set<uint256> top5_set(m_last_top5_heads.begin(), m_last_top5_heads.end());