fix ReadPreimage

[zhiqiangxu]

1. When offset>len(preimage), should return nothing instead of panic.
2. call p.GetPreimage(key) instead of p.po.GetPreimage(key) to update numPreimageRequests and totalPreimageSize.

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 85.07%. Comparing base (e80d23b) to head (7c67cea).
Report is 39 commits behind head on develop.

Files with missing lines	Patch %	Lines
cannon/mipsevm/exec/preimage.go	25.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #11805      +/-   ##
===========================================
+ Coverage    84.98%   85.07%   +0.08%     
===========================================
  Files           36       36              
  Lines         2784     2787       +3     
===========================================
+ Hits          2366     2371       +5     
+ Misses         350      347       -3     
- Partials        68       69       +1     

Flag	Coverage Δ
cannon-go-tests	85.07% <25.00%> (+0.08%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
cannon/mipsevm/exec/preimage.go	79.41% <25.00%> (+8.44%)	⬆️

[Inphi]

Good catch!