Electrs: Running with Elements Core 0.20.99 panics

Stacktrace

```

Config { log: StdErrLog { verbosity: Trace, quiet: false, show_level: true, timestamp: Off, modules: [], writer: "stderr", color_choice: Never }, network_type: LiquidRegtest, db_path: "./db/liquidregtest", daemon_dir: "/config/liquidregtest", blocks_dir: "/config/liquidregtest/blocks", daemon_rpc_addr: 172.21.0.3:18884, cookie: Some("admin1:123"), electrum_rpc_addr: 0.0.0.0:50001, http_addr: 0.0.0.0:30001, http_socket_file: None, monitoring_addr: 127.0.0.1:44224, jsonrpc_import: false, light_mode: false, address_search: false, index_unspendables: false, cors: Some("*"), precache_scripts: None, utxos_limit: 500, electrum_txs_limit: 500, electrum_banner: "Welcome to electrs-esplora 0.4.1", parent_network: Regtest, asset_db_path: None }
DEBUG - Server listening on 127.0.0.1:44224
INFO - NetworkInfo { version: 209900, subversion: "/Elements Core:0.20.99/", relayfee: 0.000001 }
DEBUG - Running accept thread
INFO - BlockchainInfo { chain: "liquidregtest", blocks: 0, headers: 0, bestblockhash: "00902a6b70c2ca83b5d9c815d96a0e2f4202179316970d14ea1847dae5b1ca21", pruned: false, verificationprogress: 0.0, initialblockdownload: Some(true) }
WARN - waiting for bitcoind sync to finish: 0/0 blocks, verification progress: 0.000%
WARN - waiting for bitcoind sync to finish: 0/0 blocks, verification progress: 0.000%
WARN - waiting for bitcoind sync to finish: 0/0 blocks, verification progress: 0.000%
WARN - waiting for bitcoind sync to finish: 0/0 blocks, verification progress: 0.000%
WARN - waiting for bitcoind sync to finish: 0/0 blocks, verification progress: 0.000%
WARN - waiting for bitcoind sync to finish: 0/0 blocks, verification progress: 0.000%
WARN - waiting for bitcoind sync to finish: 0/0 blocks, verification progress: 0.000%
DEBUG - opening DB at "./db/liquidregtest/newindex/txstore"
DEBUG - 0 blocks were added
DEBUG - opening DB at "./db/liquidregtest/newindex/history"
DEBUG - 0 blocks were indexed
DEBUG - opening DB at "./db/liquidregtest/newindex/cache"
DEBUG - downloading all block headers up to ecaf19d90b2a8bcff6468fe8837156f0702642dd355a69f5743c70915c79a00b
TRACE - downloading 2 block headers
INFO - hash=ecaf19d90b2a8bcff6468fe8837156f0702642dd355a69f5743c70915c79a00b height=1 @ 2021-09-26T13:53:11+00:00 (2 left to index)
DEBUG - adding transactions from 2 blocks using BlkFiles
DEBUG - listing block files at "/config/liquidregtest/blocks/blk*.dat"
TRACE - reading "/config/liquidregtest/blocks/blk00000.dat"
TRACE - parsing 16777216 bytes
TRACE - fetched 0 blocks
DEBUG - writing 0 rows to RocksDB { path: "./db/liquidregtest/newindex/txstore" }, flush=Disable
thread 'blkfiles_fetcher' panicked at 'failed to index 2 blocks from blk*.dat files', src/new_index/fetch.rs:143:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

```

My elements.conf

```
chain=liquidregtest
listen=1
txindex=1

[liquidregtest]
port=18886
rpcport=18884
rpcuser=admin1
rpcpassword=123
rpcallowip=0.0.0.0/0
rpcbind=0.0.0.0

mainchainrpcport=18443
mainchainrpcuser=admin1
mainchainrpcpassword=123

validatepegin=0
initialfreecoins=2100000000000000
fallbackfee=0.000001

### Dynafed

con_dyna_deploy_start=0
con_dyna_deploy_signal=1
con_nminerconfirmationwindow=1
con_nrulechangeactivationthreshold=1

```

Reproduce the panic with Elements Core 0.20.99 and the supplied liquidregtest configuration, then inspect src/new_index/fetch.rs around line 143 and the blk*.dat reading path. Use RUST_BACKTRACE=1 and verify that the block files are parsed and indexed without the fetcher panicking.

Written by the indexing model from the issue text.