NULL out handle after release

We free'd the handle but didn't NULL out the global variable, leading to
problems if you try to reinitialize the library. Make sure we clean up after
ourselves.

Signed-off-by: Dan McGee <dan@archlinux.org>

lib/libalpm/alpm.c

@@ -71,6 +71,7 @@ int SYMEXPORT alpm_release(void)
 	}
 
 	_alpm_handle_free(handle);
+	handle = NULL;
 
 	return(0);
 }