validation: don't try to invalidate genesis block

src/validation.cpp

@@ -2923,6 +2923,10 @@ bool PreciousBlock(BlockValidationState& state, const CChainParams& params, CBlo
 
 bool CChainState::InvalidateBlock(BlockValidationState& state, const CChainParams& chainparams, CBlockIndex *pindex)
 {
+    // Genesis block can't be invalidated
+    assert(pindex);
+    if (pindex->nHeight == 0) return false;
+
     CBlockIndex* to_mark_failed = pindex;
     bool pindex_was_in_chain = false;
     int disconnected = 0;