core/rawdb: fix freezer validation

[ucwong]

• core/rawdb: fix freezer validation

• core/rawdb: address comment

[codecov]

Codecov Report

Merging #1369 (5a26be7) into master (ea40615) will decrease coverage by 0.00%.
The diff coverage is 63.63%.

@@            Coverage Diff             @@
##           master    #1369      +/-   ##
==========================================
- Coverage   51.66%   51.65%   -0.01%     
==========================================
  Files         461      461              
  Lines       58044    58048       +4     
==========================================
- Hits        29990    29987       -3     
- Misses      25785    25792       +7     
  Partials     2269     2269              

Impacted Files	Coverage Δ
core/rawdb/freezer.go	42.61% <63.63%> (-0.31%)	⬇️
rpc/ipc.go	68.75% <0.00%> (-12.50%)	⬇️
rpc/server.go	77.58% <0.00%> (-6.90%)	⬇️
rlp/raw.go	82.38% <0.00%> (-4.67%)	⬇️
rpc/client.go	85.38% <0.00%> (-1.00%)	⬇️
p2p/peer.go	74.16% <0.00%> (-0.84%)	⬇️
ctxc/downloader/queue.go	80.00% <0.00%> (-0.47%)	⬇️
p2p/discover/v5_udp.go	81.15% <0.00%> (-0.23%)	⬇️
accounts/keystore/watch.go	95.83% <0.00%> (ø)
ctxc/downloader/downloader.go	76.86% <0.00%> (+0.31%)	⬆️
... and 5 more