Data Race in Execute

[egonelbre]

Running tests with go test -race -count 5 -parallel 32, shows:

==================
WARNING: DATA RACE
Write at 0x00c000007138 by goroutine 104:
  github.com/graphql-go/graphql.Execute.func1()
      F:/Go/src/github.com/graphql-go/graphql/executor.go:61 +0x333

Previous read at 0x00c000007138 by goroutine 43:
  github.com/graphql-go/graphql.Execute()
      F:/Go/src/github.com/graphql-go/graphql/executor.go:75 +0x345
  github.com/graphql-go/graphql.Do()
      F:/Go/src/github.com/graphql-go/graphql/graphql.go:55 +0x664
  github.com/graphql-go/graphql_test.TestContextDeadline()
      F:/Go/src/github.com/graphql-go/graphql/executor_test.go:1895 +0x5c8
  testing.tRunner()
      C:/Go/src/testing/testing.go:865 +0x16a

Goroutine 104 (running) created at:
  github.com/graphql-go/graphql.Execute()
      F:/Go/src/github.com/graphql-go/graphql/executor.go:36 +0x215
  github.com/graphql-go/graphql.Do()
      F:/Go/src/github.com/graphql-go/graphql/graphql.go:55 +0x664
  github.com/graphql-go/graphql_test.TestContextDeadline()
      F:/Go/src/github.com/graphql-go/graphql/executor_test.go:1895 +0x5c8
  testing.tRunner()
      C:/Go/src/testing/testing.go:865 +0x16a
==================
--- FAIL: TestValidate_OverlappingFieldsCanBeMerged_ReturnTypesMustBeUnambiguous_ReportsCorrectlyWhenANonExclusiveFollosAnExclusive (0.02s)