Unhandled exception in callback when connection is disposed

Read callback (in SSL mode) may throw exception if connection is disposed. User code cannot catch and handle this exception resulting in process being terminated. This fix resolved the issue.

Below is the example stack we observed. We also observed NullReferenceException (depending on the timing), 

Unhandled Exception: System.ObjectDisposedException: Cannot access a disposed object.
Object name: 'SslStream'.
   at System.Net.Security.SslState.CheckThrow(Boolean authSucessCheck)
   at System.Net.Security.SslStream.BeginRead(Byte[] buffer, Int32 offset, Int32 count, AsyncCallback asyncCallback, Object asyncState)
   at StackExchange.Redis.PhysicalConnection.BeginReading()
   at System.Net.LazyAsyncResult.Complete(IntPtr userToken)
   at System.Net.Security._SslStream.ProcessFrameBody(Int32 readBytes, Byte[] buffer, Int32 offset, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security._SslStream.ReadFrameCallback(AsyncProtocolRequest asyncRequest)
   at System.Net.FixedSizeReader.CheckCompletionBeforeNextRead(Int32 bytes)
   at System.Net.FixedSizeReader.ReadCallback(IAsyncResult transportResult)
   at System.Net.LazyAsyncResult.Complete(IntPtr userToken)
   at System.Net.Security._SslStream.ProcessFrameBody(Int32 readBytes, Byte[] buffer, Int32 offset, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security._SslStream.ReadFrameCallback(AsyncProtocolRequest asyncRequest)
   at System.Net.FixedSizeReader.CheckCompletionBeforeNextRead(Int32 bytes)
   at System.Net.FixedSizeReader.ReadCallback(IAsyncResult transportResult)
   at System.Net.LazyAsyncResult.Complete(IntPtr userToken)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Net.LazyAsyncResult.Complete(IntPtr userToken)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Net.ContextAwareResult.Complete(IntPtr userToken)
   at System.Net.Sockets.BaseOverlappedAsyncResult.CompletionPortCallback(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* nativeOverlapped)
   at System.Threading._IOCompletionCallback.PerformIOCompletionCallback(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* pOVERLAP)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Net.ContextAwareResult.Complete(IntPtr userToken)
   at System.Net.Sockets.BaseOverlappedAsyncResult.CompletionPortCallback(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* nativeOverlapped)
   at System.Threading._IOCompletionCallback.PerformIOCompletionCallback(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* pOVERLAP)
Here