Fixed: SymWriter COM object is not released on exception

The Problem:

Problem observed when being used in coverlet:
coverlet-coverage/coverlet#1471
If an exception caused `NativePdbWriter.Write` to never be
called, it would not call `SymWriter.Close`,
which in turn meant `Marshal.ReleaseComObject` was left uncalled.

The garbage collector will eventually destroy the object
and thereby release the COM object, but that happens non-deterministically.
The COM object is holding to a file handle, which prevents
other operations on the written file until it is released.
The result was random file access issues.

The Solution:

Luckily NativePdbWriter is IDisposable. I added a call to
`writer.Close` there. But now it could be called twice,
so I had to add a boolean to SymWriter to avoid releasing
everything twice.

symbols/pdb/Mono.Cecil.Pdb/NativePdbWriter.cs

@@ -267,6 +267,7 @@ public void Write ()
 
 		public void Dispose ()
 		{
+			writer.Close ();
 		}
 	}
 

symbols/pdb/Mono.Cecil.Pdb/SymWriter.cs

@@ -30,6 +30,7 @@ static extern int CoCreateInstance (
 
 		readonly ISymUnmanagedWriter2 writer;
 		readonly Collection<ISymUnmanagedDocumentWriter> documents;
+		bool closed = false;
 
 		public SymWriter ()
 		{
@@ -78,11 +79,14 @@ public void DefineConstant2 (string name, object value, int sigToken)
 
 		public void Close ()
 		{
-			writer.Close ();
-			Marshal.ReleaseComObject (writer);
+			if( !closed ) {
+				closed = true;
+				writer.Close ();
+				Marshal.ReleaseComObject ( writer );
 
-			foreach (var document in documents)
-				Marshal.ReleaseComObject (document);
+				foreach( var document in documents )
+					Marshal.ReleaseComObject ( document );
+			}
 		}
 
 		public void CloseMethod ()