bdiff: document that bdiff_freehunks() accepts NULL

bdiff: document that bdiff_freehunks() accepts NULL

blocks() of cext/bdiff.c may pass NULL on OOM.

--- a/mercurial/bdiff.c	Thu Jun 14 13:44:42 2018 +0200
+++ b/mercurial/bdiff.c	Thu Jun 14 20:25:16 2018 +0900
@@ -310,6 +310,7 @@
 	return count;
 }
 
+/* deallocate list of hunks; l may be NULL */
 void bdiff_freehunks(struct bdiff_hunk *l)
 {
 	struct bdiff_hunk *n;