Check before allocate

Author: Stephen Hawley

SwiftRuntimeLibrary/SwiftMarshal/StructMarshal.cs

@@ -25,9 +25,9 @@ public static unsafe IntPtr RetainSwiftObject (ISwiftObject obj)
 		{
 			if (obj == null)
 				return IntPtr.Zero;
-			byte* valueBuffer = stackalloc byte [3 * IntPtr.Size];
 			if (obj.SwiftObject == IntPtr.Zero)
 				throw new SwiftRuntimeException ("SwiftObject handle is IntPtr.Zero, likely because it was disposed.");
+			byte* valueBuffer = stackalloc byte [3 * IntPtr.Size];
 			SwiftCore.swift_beginAccess (obj.SwiftObject, valueBuffer, (uint)SwiftExclusivityFlags.Track, IntPtr.Zero);
 			var result = SwiftCore.Retain (obj.SwiftObject);
 			SwiftCore.swift_endAccess (valueBuffer);