Fix UB in hash-map.h

bootstrap-ubsan shows:
  gcc/hash-map.h:277:19: runtime error: member access within null pointer of type 'struct hash_map'

Fix the issue by returning early.

From-SVN: r255166

gcc/ChangeLog

+2017-11-27  Markus Trippelsdorf  <markus@trippelsdorf.de>
+
+	* hash-map.h (gt_cleare_cache): Avoid UB.
+
 2017-11-27  Eric Botcazou  <ebotcazou@adacore.com>
 
 	* cfgloop.h (struct loop): Document usage of USHRT_MAX for unroll.

gcc/hash-map.h

@@ -274,7 +274,8 @@ template<typename K, typename V, typename H>
 static inline void
 gt_cleare_cache (hash_map<K, V, H> *h)
 {
-  gt_cleare_cache (&h->m_table);
+  if (h)
+    gt_cleare_cache (&h->m_table);
 }
 
 template<typename K, typename V, typename H>