Fix: multimap shouldn't assume it owns the connection

Dapper NET40/SqlMapper.cs

@@ -1073,7 +1073,7 @@ partial class DontMap { }
             IDbCommand ownedCommand = null;
             IDataReader ownedReader = null;
 
-            bool wasClosed = cnn.State == ConnectionState.Closed;
+            bool wasClosed = cnn != null && cnn.State == ConnectionState.Closed;
             try
             {
                 if (reader == null)