Added a global string to be used for the tempdir. since the user preference is not loaded in background mode and the user preference is not validated and has no fallback.

'btempdir' is set with BLI_where_is_temp() - This tries to use U.tempdir but falls back to $TEMP or /tmp/

source/blender/python/api2_2x/Blender.c

@@ -286,6 +286,7 @@ static PyObject *Blender_Set( PyObject * self, PyObject * args )
 		if ( !PyArg_Parse( arg , "s" , &dir ))
 			return EXPP_ReturnPyObjError( PyExc_ValueError, "expected a string" );
 		BLI_strncpy(U.tempdir, dir, FILE_MAXDIR);
+		BLI_where_is_temp( btempdir, 1 );
 	} else if (StringEqual( name , "compressfile" ) ) {
 		int value = PyObject_IsTrue( arg );