Add null checks to MobileAuthenticatedStream to avoid null reference …

…exception

When attached with the native debugger, it does not allow you to continue past the native signal raised due to the null refrence exception.

mcs/class/System/Mono.Net.Security/MobileAuthenticatedStream.cs

@@ -725,6 +725,9 @@ internal AsyncOperationStatus ProcessHandshake (AsyncOperationStatus status, boo
 		internal (int ret, bool wantMore) ProcessRead (BufferOffsetSize userBuffer)
 		{
 			lock (ioLock) {
+				if (xobileTlsContext == null)
+					return (0, false);
+
 				// This operates on the internal buffer and will never block.
 				if (operation != Operation.Authenticated)
 					throw GetInternalError ();
@@ -740,6 +743,9 @@ internal AsyncOperationStatus ProcessHandshake (AsyncOperationStatus status, boo
 		internal (int ret, bool wantMore) ProcessWrite (BufferOffsetSize userBuffer)
 		{
 			lock (ioLock) {
+				if (xobileTlsContext == null)
+					return (0, false);
+
 				// This operates on the internal buffer and will never block.
 				if (operation != Operation.Authenticated)
 					throw GetInternalError ();