MDEV-33478: Tests massively fail with clang-18 -fsanitize=memory

Starting with clang-16, MemorySanitizer appears to check that
uninitialized values not be passed by value nor returned.
Previously, it was allowed to copy uninitialized data in such cases.

get_foreign_key_info(): Remove a local variable that was passed
uninitialized to a function.

DsMrr_impl: Initialize key_buffer, because DsMrr_impl::dsmrr_init()
is reading it.

test_bind_result_ext1(): MYSQL_TYPE_LONG is 32 bits, hence we must
use a 32-bit type, such as int. sizeof(long) differs between
LP64 and LLP64 targets.

sql/debug_sync.cc

@@ -1008,7 +1008,7 @@ static bool debug_sync_eval_action(THD *thd, char *action_str, char *action_end)
   st_debug_sync_action  *action= NULL;
   const char            *errmsg;
   char                  *ptr;
-  char                  *token;
+  char                  *token= nullptr;
   uint                  token_length= 0;
   DBUG_ENTER("debug_sync_eval_action");
   DBUG_ASSERT(thd);