Fix Bug#59307 Valgrind: uninitialized value in rw_lock_set_writer_id_and_recursion_flag()

by silencing a bogus Valgrind warning:

==4392== Conditional jump or move depends on uninitialised value(s)
==4392==    at 0x5A18416: rw_lock_set_writer_id_and_recursion_flag (sync0rw.ic:283)
==4392==    by 0x5A1865C: rw_lock_x_lock_low (sync0rw.c:558)
==4392==    by 0x5A18481: rw_lock_x_lock_func (sync0rw.c:617)
==4392==    by 0x597EEE6: mtr_x_lock_func (mtr0mtr.ic:271)
==4392==    by 0x597EBBD: fsp_header_init (fsp0fsp.c:970)
==4392==    by 0x5A15E78: innobase_start_or_create_for_mysql (srv0start.c:1508)
==4392==    by 0x598B789: innobase_init(void*) (ha_innodb.cc:2282)

os_compare_and_swap_thread_id() is defined as
__sync_bool_compare_and_swap(). From the GCC doc:

`bool __sync_bool_compare_and_swap (TYPE *ptr, TYPE oldval TYPE newval, ...)'
  ...
  The "bool" version returns true if the comparison is successful and
  NEWVAL was written.

So it is not possible that the return value is uninitialized, no matter what
the arguments to os_compare_and_swap_thread_id() are. Probably Valgrind gets
confused by the implementation of the GCC internal function
__sync_bool_compare_and_swap().

storage/innodb_plugin/include/sync0rw.ic

@@ -280,6 +280,7 @@ rw_lock_set_writer_id_and_recursion_flag(
 	local_thread = lock->writer_thread;
 	success = os_compare_and_swap_thread_id(
 		&lock->writer_thread, local_thread, curr_thread);
+	UNIV_MEM_VALID(&success, sizeof(success));
 	ut_a(success);
 	lock->recursive = recursive;