Fix small memory leaks induced by diff_tree_setup_paths

Run diff_tree_release_paths in the appropriate places, and add a test to
avoid NULL dereference. Better safe than sorry.

Signed-off-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-reset.c

@@ -158,6 +158,7 @@ static int read_from_tree(const char *prefix, const char **argv,
 		return 1;
 	diffcore_std(&opt);
 	diff_flush(&opt);
+	diff_tree_release_paths(&opt);
 
 	if (!index_was_discarded)
 		/* The index is still clobbered from do_diff_cache() */