don't crash on thread detach - now Diablo 2 videotest isn't crashing anymore

svn path=/trunk/; revision=31763

reactos/dll/win32/opengl32/opengl32.c

@@ -91,6 +91,7 @@ OPENGL32_ThreadDetach()
 		if (!HeapFree( GetProcessHeap(), 0, lpData ))
 			DBGPRINT( "Warning: HeapFree() on GLTHREADDATA failed (%d)",
 			          GetLastError() );
+		lpData = NULL;
 	}
 
 	dispatchTable = NtCurrentTeb()->glTable;

reactos/dll/win32/opengl32/wgl.c

@@ -978,6 +978,9 @@ rosglMakeCurrent( HDC hdc, HGLRC hglrc )
 
 	DBGTRACE( "Called!" );
 
+	if (OPENGL32_threaddata == NULL)
+		return FALSE;
+
 	/* flush current context */
 	if (OPENGL32_threaddata->glrc != NULL)
 	{