fix: lock before checking sp.rendered to avoid datarace

[jvogit]

We are using libopenapi + vacuum for our internal tooling. We've run into data race in our unit tests:

==================
WARNING: DATA RACE
Write at 0x00c000520198 by goroutine 305:
  github.com/pb33f/libopenapi/datamodel/high/base.(*SchemaProxy).Schema()
      project/vendor/github.com/pb33f/libopenapi/datamodel/high/base/schema_proxy.go:146 +0xbb9
  github.com/pb33f/doctor/model/high/base.(*SchemaProxy).Walk()
      project/vendor/github.com/pb33f/doctor/model/high/base/schema_proxy.go:80 +0xfd
  github.com/pb33f/doctor/model/high/v3.(*MediaType).Walk.func1()
      project/vendor/github.com/pb33f/doctor/model/high/v3/media_type.go:39 +0x65
  github.com/sourcegraph/conc/panics.(*Catcher).Try()
      project/vendor/github.com/sourcegraph/conc/panics/panics.go:23 +0x77
  github.com/sourcegraph/conc.(*WaitGroup).Go.func1()
      project/vendor/github.com/sourcegraph/conc/waitgroup.go:32 +0x8f

Previous read at 0x00c000520198 by goroutine 296:
  github.com/pb33f/libopenapi/datamodel/high/base.(*SchemaProxy).Schema()
      project/vendor/github.com/pb33f/libopenapi/datamodel/high/base/schema_proxy.go:97 +0x85
  github.com/pb33f/doctor/model/high/v3.(*MediaType).GetSize()
      project/vendor/github.com/pb33f/doctor/model/high/v3/media_type.go:123 +0x304
  github.com/pb33f/doctor/model.(*DrDocument).processObject()
      project/vendor/github.com/pb33f/doctor/model/walk_model.go:602 +0xfc8
  github.com/pb33f/doctor/model.(*DrDocument).walkV3.func1()
      project/vendor/github.com/pb33f/doctor/model/walk_model.go:457 +0x604
  github.com/pb33f/doctor/model.(*DrDocument).walkV3.gowrap1()
      project/vendor/github.com/pb33f/doctor/model/walk_model.go:466 +0x61

A concurrent read and write is done on those lines, specifically with sp.rendered. This fixes this by deferring the unlock and moving sp.rendered into the critical section.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.62%. Comparing base (d3dcac4) to head (f6f58b4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #391      +/-   ##
==========================================
+ Coverage   99.55%   99.62%   +0.06%     
==========================================
  Files         165      165              
  Lines       21606    21603       -3     
==========================================
+ Hits        21509    21521      +12     
+ Misses         92       77      -15     
  Partials        5        5              

Flag	Coverage Δ
unittests	99.62% <100.00%> (+0.06%)	⬆️

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[daveshanley]

looks good to me! Thank you for tuning this up!