validation: add missing insert to m_dirty_blockindex

...in FindMostWorkChain(). Before this, it was possible that the change
to the block index wouldn't be persisted to disk.

src/validation.cpp

@@ -2914,6 +2914,7 @@ CBlockIndex* Chainstate::FindMostWorkChain()
                 while (pindexTest != pindexFailed) {
                     if (fFailedChain) {
                         pindexFailed->nStatus |= BLOCK_FAILED_CHILD;
+                        m_blockman.m_dirty_blockindex.insert(pindexFailed);
                     } else if (fMissingData) {
                         // If we're missing data, then add back to m_blocks_unlinked,
                         // so that if the block arrives in the future we can try adding