valgrind failures

and a couple of collaterals:
* debugging assert in my_valgrind.h
* trivial cleanup in sql_union.cc

include/my_valgrind.h

@@ -41,3 +41,5 @@
 #define TRASH_FREE(A,B) TRASH_FILL(A,B,0x8F)
 #define TRASH(A,B) TRASH_FREE(A,B)
 
+# define DBUG_ASSERT_DEFINED(x) \
+  DBUG_ASSERT(!VALGRIND_CHECK_MEM_IS_DEFINED(&(x), sizeof(x)))

plugin/file_key_management/parser.cc

@@ -333,7 +333,8 @@ char* Parser::read_and_decrypt_file(const char *secret)
 
 // Check for file encryption
   uchar *decrypted;
-  if (is_prefix((char*)buffer, OpenSSL_prefix))
+  if (file_size > OpenSSL_prefix_len &&
+      is_prefix((char*)buffer, OpenSSL_prefix))
   {
     uchar key[OpenSSL_key_len];
     uchar iv[OpenSSL_iv_len];

sql/item_strfunc.cc

@@ -572,7 +572,7 @@ String *Item_func_decode_histogram::val_str(String *str)
   uint i;
   str->length(0);
   char numbuf[32];
-  const uchar *p= (uchar*)res->c_ptr();
+  const uchar *p= (uchar*)res->c_ptr_safe();
   for (i= 0; i < res->length(); i++)
   {
     double val;

sql/sql_select.cc

@@ -2648,7 +2648,7 @@ bool JOIN::make_aggr_tables_info()
     */
     DBUG_PRINT("info",("Sorting for order by/group by"));
     ORDER *order_arg= group_list ?  group_list : order;
-    if (join_tab &&
+    if (top_join_tab_count + aggr_tables > const_tables &&
         ordered_index_usage !=
         (group_list ? ordered_index_group_by : ordered_index_order_by) &&
         curr_tab->type != JT_CONST &&

sql/sql_union.cc

@@ -445,9 +445,10 @@ bool st_select_lex_unit::prepare(THD *thd_arg, select_result *sel_result,
   bool is_union_select;
   bool instantiate_tmp_table= false;
   DBUG_ENTER("st_select_lex_unit::prepare");
-  DBUG_ASSERT(thd == thd_arg && thd == current_thd);
+  DBUG_ASSERT(thd == thd_arg);
+  DBUG_ASSERT(thd == current_thd);
 
-  describe= MY_TEST(additional_options & SELECT_DESCRIBE);
+  describe= additional_options & SELECT_DESCRIBE;
 
   /*
     Save fake_select_lex in case we don't need it for anything but