libstdc++: Fix null dereference in pb_ds containers

This fixes ubsan errors:

ext/pb_ds/detail/cc_hash_table_map_/cc_ht_map_.hpp:533:15: runtime error: member access within null pointer of type 'struct entry'

libstdc++-v3/ChangeLog:

	* include/ext/pb_ds/detail/cc_hash_table_map_/cc_ht_map_.hpp
	(find_key_pointer(key_const_reference, false_type))
	(find_key_pointer(key_const_reference, true_type)): Do not
	dereference null pointer.

libstdc++-v3/include/ext/pb_ds/detail/cc_hash_table_map_/cc_ht_map_.hpp

@@ -524,13 +524,16 @@ namespace __gnu_pbds
 
 	resize_base::notify_find_search_end();
 
-#ifdef _GLIBCXX_DEBUG
 	if (p_e == 0)
-	  PB_DS_CHECK_KEY_DOES_NOT_EXIST(r_key)
+	  {
+	    PB_DS_CHECK_KEY_DOES_NOT_EXIST(r_key)
+	    return 0;
+	  }
 	else
-	  PB_DS_CHECK_KEY_EXISTS(r_key)
-#endif
-	return &p_e->m_value;
+	  {
+	    PB_DS_CHECK_KEY_EXISTS(r_key)
+	    return &p_e->m_value;
+	  }
       }
 
       inline pointer
@@ -550,13 +553,16 @@ namespace __gnu_pbds
 
 	resize_base::notify_find_search_end();
 
-#ifdef _GLIBCXX_DEBUG
 	if (p_e == 0)
-	  PB_DS_CHECK_KEY_DOES_NOT_EXIST(r_key)
+	  {
+	    PB_DS_CHECK_KEY_DOES_NOT_EXIST(r_key)
+	    return 0;
+	  }
 	else
-	  PB_DS_CHECK_KEY_EXISTS(r_key)
-#endif
-	return &p_e->m_value;
+	  {
+	    PB_DS_CHECK_KEY_EXISTS(r_key)
+	    return &p_e->m_value;
+	  }
       }
 
       inline bool