Make sk_sort tolearate a NULL argument.

CHANGES

@@ -3,6 +3,9 @@
 
  Changes between 0.9.6 and 0.9.7  [xx XXX 2000]
 
+  *) Make sk_sort() tolerate a NULL argument.
+     [Steve Henson reported by Massimiliano Pala <madwolf@comune.modena.it>]
+
   *) New OCSP verify flag OCSP_TRUSTOTHER. When set the "other" certificates
      passed by the function are trusted implicitly. If any of them signed the
      reponse then it is assumed to be valid and is not verified.

crypto/stack/stack.c

@@ -316,7 +316,7 @@ char *sk_set(STACK *st, int i, char *value)
 
 void sk_sort(STACK *st)
 	{
-	if (!st->sorted)
+	if (st && !st->sorted)
 		{
 		int (*comp_func)(const void *,const void *);