Unnecessary lock operation for reading from cache in extractStructCache method - fixed double checknig/reading cStruct in validateStruct method

[amirkaramiit]

Fixes Or Enhances

• Fixed unnecessary Lock, and UnLock Operations in *Validate.extractStructCache method while the struct (cStruct) has been cached.
• Fixed double-checking structCache for a Cached struct in *validate.validateStruct method, while it was done in *Validate.extractStructCache method.

Make sure that you've checked the boxes below before you submit PR:

• Tests exist or have been written that cover this particular change.

@go-playground/validator-maintainers

[coveralls]

coverage: 74.167% (-0.005%) from 74.172%
when pulling e83b403 on amirkaramiit:validator-extractCStruct-get-duplicate
into b328f72 on go-playground:master.