Fix for #1217

[chrfin]

This change should fix #1217
I am aware that GetOrAdd is not thread-safe, but according to the list under remarks at https://docs.microsoft.com/en-us/dotnet/api/system.collections.concurrent.concurrentdictionary-2.getoradd it should not cause any problems in this case.

[Seabizkit]

Maybe i was a bit quick with this, and do not know how to delete now. sorry.

[kosinsky]

In our logs I found that it fails with Null ref exception in the same place as in #1217.
Looks like this PR fixes it too. but just in case full stack trace:

System.NullReferenceException: Object reference not set to an instance of an object.
   at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at System.Data.Entity.Core.Query.InternalTrees.BitVec.ArrayPool.GetBag(Int32 length)
   at System.Data.Entity.Core.Query.InternalTrees.BitVec.ArrayPool.GetArray(Int32 length)
   at System.Data.Entity.Core.Query.InternalTrees.BitVec..ctor(Int32 length, Boolean defaultValue)
   at System.Data.Entity.Core.Query.InternalTrees.Command.CreateVarVec()
   at System.Data.Entity.Core.Query.InternalTrees.Table..ctor(Command command, TableMD tableMetadata, Int32 tableId)
   at System.Data.Entity.Core.Query.InternalTrees.Command.CreateTableInstance(TableMD tableMetadata)
   at System.Data.Entity.Core.Query.PlanCompiler.ITreeGenerator.Visit(DbScanExpression e)
   at System.Data.Entity.Core.Query.PlanCompiler.ITreeGenerator.VisitBoundExpression(DbExpression boundExpression, Var& boundVar)
   at System.Data.Entity.Core.Query.PlanCompiler.ITreeGenerator.EnterExpressionBinding(DbExpressionBinding binding)
   at System.Data.Entity.Core.Query.PlanCompiler.ITreeGenerator.Visit(DbFilterExpression e)
   at System.Data.Entity.Core.Query.PlanCompiler.ITreeGenerator..ctor(DbQueryCommandTree ctree, DiscriminatorMap discriminatorMap)
   at System.Data.Entity.Core.Query.PlanCompiler.PlanCompiler.Initialize()

[ajcvickers]

@chrfin Can you re-submit this against release/6.4?

[chrfin]

No problem, will do so...

[chrfin]

@ajcvickers Please see #1426