index: Fix backwards search for bestblock

This allows filters to be reconstructed when the best known block is
the Genesis block without needing to reindex.
It fixes Init errors seen in bitcoin#23289.

Author: mzumsande

src/index/base.cpp

@@ -91,11 +91,12 @@ bool BaseIndex::Init()
             const CBlockIndex* block = active_chain.Tip();
             prune_violation = true;
             // check backwards from the tip if we have all block data until we reach the indexes bestblock
-            while (block_to_test && block->pprev && (block->pprev->nStatus & BLOCK_HAVE_DATA)) {
+            while (block_to_test && block && (block->nStatus & BLOCK_HAVE_DATA)) {
                 if (block_to_test == block) {
                     prune_violation = false;
                     break;
                 }
+                assert(block->pprev);
                 block = block->pprev;
             }
         }