MDEV-18878: Fix GCC -flifetime-dse

GCC 6 and later can optimize away the memset() that is part of
mem_heap_zalloc() in a placement new call. So, instead of relying
on that kind of initialization, explicitly initialize the necessary
fields in the constructors.

que_common_t::que_common_t(): Initialize more fields in the
default constructor.

purge_vcol_info_t::purge_vcol_info_t(): Initialize all fields in
the default constructor.

purge_node_t::purge_node_t(): Initialize all necessary fields.

Reference:

    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71388

    https://gcc.gnu.org/ml/gcc/2016-02/msg00207.html

storage/innobase/include/que0types.h

@@ -87,8 +87,9 @@ struct que_common_t{
 				explicitly */
 
 	/** Constructor */
-	que_common_t(ulint type, que_node_t* parent)
-		: type(type), parent(parent), brother(), val(), val_buf_size()
+	que_common_t(ulint type, que_node_t* parent) :
+		type(type), parent(parent), brother(NULL),
+		val(), val_buf_size(0)
 	{}
 };
 

storage/innobase/include/row0purge.h

@@ -133,7 +133,14 @@ struct purge_node_t{
 
 	/** Constructor */
 	explicit purge_node_t(que_thr_t* parent) :
-		common(QUE_NODE_PURGE, parent), heap(mem_heap_create(256))
+		common(QUE_NODE_PURGE, parent),
+		undo_recs(NULL),
+		unavailable_table_id(0),
+		heap(mem_heap_create(256)),
+#ifdef UNIV_DEBUG
+		in_progress(false),
+#endif
+		vcol_info()
 	{}
 
 #ifdef UNIV_DEBUG

storage/innobase/include/row0types.h

@@ -69,6 +69,11 @@ struct purge_vcol_info_t
 	TABLE*	mariadb_table;
 
 public:
+	/** Default constructor */
+	purge_vcol_info_t() :
+		requested(false), used(false), first_use(false),
+		mariadb_table(NULL)
+	{}
 	/** Reset the state. */
 	void reset()
 	{

storage/innobase/trx/trx0purge.cc

@@ -182,7 +182,7 @@ purge_graph_build()
 
 	for (ulint i = 0; i < srv_n_purge_threads; ++i) {
 		que_thr_t*	thr = que_thr_create(fork, heap, NULL);
-		thr->child = new(mem_heap_zalloc(heap, sizeof(purge_node_t)))
+		thr->child = new(mem_heap_alloc(heap, sizeof(purge_node_t)))
 			purge_node_t(thr);
 	}