CSHARP-2339: Only call AbortTransaction from Dispose when appropriate.

Author: rstam

src/MongoDB.Driver.Core/Core/Bindings/CoreSession.cs

@@ -322,13 +322,19 @@ public void Dispose()
             {
                 if (_currentTransaction != null)
                 {
-                    try
-                    {
-                        AbortTransaction(CancellationToken.None);
-                    }
-                    catch
+                    switch (_currentTransaction.State)
                     {
-                        // ignore exceptions
+                        case CoreTransactionState.Starting:
+                        case CoreTransactionState.InProgress:
+                            try
+                            {
+                                AbortTransaction(CancellationToken.None);
+                            }
+                            catch
+                            {
+                                // ignore exceptions
+                            }
+                            break;
                     }
                 }