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 #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;
             }
         }