Fixed forgotten mutexes Unlock calls

[iulianpascalau]

Reasoning behind the pull request

• fixed forgotten mutex.Unlock calls

Proposed changes

Testing procedure

Pre-requisites

Based on the Contributing Guidelines the PR author and the reviewers must check the following requirements are met:

• was the PR targeted to the correct branch?
• if this is a larger feature that probably needs more than one PR, is there a feat branch created?
• if this is a feat branch merging, do all satellite projects have a proper tag inside go.mod?

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (4c22494) 80.17% compared to head (dd92772) 80.17%.

Files	Patch %	Lines
process/block/metablock.go	50.00%	1 Missing ⚠️
process/block/shardblock.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           rc/v1.6.0    #5761      +/-   ##
=============================================
- Coverage      80.17%   80.17%   -0.01%     
=============================================
  Files            708      708              
  Lines          94005    94007       +2     
=============================================
  Hits           75370    75370              
- Misses         13291    13293       +2     
  Partials        5344     5344              

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[ovidiuolteanu]

warning no longer present in static analysis with this fix

[gabi-vuls]

Normal allin test: master-e134e5314d -> fix-forgotten-mutex-unlock-dd92772c4d

--- Specific errors ---

block hash does not match 2573
wrong nonce in block 1088
miniblocks does not match 0
num miniblocks does not match 0
miniblock hash does not match 0
block bodies does not match 0
receipts hash missmatch 0

/------/

--- Statistics ---

Nr. of all ERRORS: 0
Nr. of all WARNS: 406
Nr. of new ERRORS: 0
Nr. of new WARNS: 4
Nr. of PANICS: 0

/------/

--- ERRORS ---

/------/

--- WARNINGS ---

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