Fixed: SymWriter COM object is not released on exception (#955)

* 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.

* Code style

---------

Co-authored-by: Jb Evain <jb@evain.net>

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 ()
 		{
@@ -78,6 +79,10 @@ public void DefineConstant2 (string name, object value, int sigToken)
 
 		public void Close ()
 		{
+			if (closed)
+				return;
+
+			closed = true;
 			writer.Close ();
 			Marshal.ReleaseComObject (writer);