bdiff: document that bdiff_freehunks() accepts NULL

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

mercurial/bdiff.c

@@ -310,6 +310,7 int bdiff_diff(struct bdiff_line *a, int
 	return count;
 }
 
+/* deallocate list of hunks; l may be NULL */
 void bdiff_freehunks(struct bdiff_hunk *l)
 {
 	struct bdiff_hunk *n;