blockstorage: check nPos in ReadRawBlockFromDisk before seeking back

ReadRawBlockFromDisk assumes a non-null pos that has an nPos >= 8.
This simple check makes the function safer to call in the future,
so callers don't need to worry about causing UB if the pos is null.

Author: andrewtoth

src/node/blockstorage.cpp

@@ -1088,6 +1088,12 @@ bool BlockManager::ReadBlockFromDisk(CBlock& block, const CBlockIndex& index) co
 bool BlockManager::ReadRawBlockFromDisk(std::vector<uint8_t>& block, const FlatFilePos& pos) const
 {
     FlatFilePos hpos = pos;
+    // If nPos is less than 8 the pos is null and we don't have the block data
+    // Return early to prevent undefined behavior of unsigned int underflow
+    if (hpos.nPos < 8) {
+        LogError("%s: OpenBlockFile failed for %s\n", __func__, pos.ToString());
+        return false;
+    }
     hpos.nPos -= 8; // Seek back 8 bytes for meta header
     AutoFile filein{OpenBlockFile(hpos, true)};
     if (filein.IsNull()) {