fix: null pointer bug with rpc interface

[wenchaopeng]

• fix ledger process block may case painc when call map "checkBlockFns"  #333

Signed-off-by: wenchao 659672152@qq.com

Proposed changes in this pull request

Explain the changes in this pull request in order to help the project maintainers understand the overall impact of it.

Please put an x against the checkboxes.

Type

• Bug fix: (Link to the issue #{issue No.})
• Feature (Non-breaking change)
• Feature (Breaking change)
• Documentation Improvement

Checklist

Write a small comment explaining if its N/A (not applicable)

• Read the CONTRIBUTION.
• All the tests are passing after the introduction of new changes.
• Added tests respective to the part of code I have written.
• Added proper documentation where ever applicable.
• Code has been written according to Golang-Style-Guide

Extra information

Any extra information related to this pull request.

[codecov]

Codecov Report

Merging #405 into master will increase coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #405      +/-   ##
==========================================
+ Coverage    44.1%   44.11%   +<.01%     
==========================================
  Files         168      168              
  Lines       17221    17313      +92     
==========================================
+ Hits         7596     7637      +41     
- Misses       8422     8458      +36     
- Partials     1203     1218      +15

Impacted Files	Coverage Δ
rpc/api/ledger.go	0% <0%> (ø)	⬆️
rpc/api/rewards.go	47.61% <0%> (-5.02%)	⬇️
rpc/api/mintage.go	0% <0%> (ø)	⬆️
rpc/api/nep5_pledge.go	0% <0%> (ø)	⬆️
common/sync/hashmap/list.go	78.65% <0%> (-2.25%)	⬇️
p2p/sync.go	7.01% <0%> (-1.05%)	⬇️
rpc/utils.go	89% <0%> (-1%)	⬇️
common/sync/hashmap/hashmap.go	94.88% <0%> (-0.94%)	⬇️
p2p/streamManager.go	78.23% <0%> (+2.17%)	⬆️
common/time.go	90.47% <0%> (+4.76%)	⬆️
... and 2 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 701cfd2...e5fb7b7. Read the comment docs.

[gythialy]

if private key is nil, will not sign the block and it should be OK