libstdc++: Fix null dereferences in std::promise

libstdc++: Fix null dereferences in std::promise

This fixes some ubsan errors in std::promise:

future:1153:34: runtime error: member call on null pointer of type 'struct element_type'
future:1153:34: runtime error: member access within null pointer of type 'struct element_type'

The problem is that the check for a null pointer is done inside the
_State::__Setter function, which is only evaluated after evaluating the
_M_future->_M_set_result postfix-expression.

This change adds a new promise::_M_state() helper for accessing
_M_future, and moves the check for no shared state into there, instead
of inside the __setter functions. The __setter functions are made
always_inline, to avoid the situation where the linker selects the old
version of set_value (without the _S_check call) and the new version of
__setter (without the _S_check call) and so there is no check. With the
always_inline attribute the old version of set_value will either inline
the old __setter or call an extern definition of it, and the new
set_value will do the check itself, so both versions do the check.

libstdc++-v3/ChangeLog:

	* include/std/future (promise::set_value): Check for existence
	of shared state before dereferncing it.
	(promise::set_exception, promise::set_value_at_thread_exit)
	(promise::set_exception_at_thread_exit): Likewise.
	(promise<R&>::set_value, promise<R&>::set_exception)
	(promise<R&>::set_value_at_thread_exit)
	(promise<R&>::set_exception_at_thread_exit): Likewise.
	(promise<void>::set_value, promise<void>::set_exception)
	(promise<void>::set_value_at_thread_exit)
	(promise<void>::set_exception_at_thread_exit): Likewise.
	* testsuite/30_threads/promise/members/at_thread_exit2.cc:
	Remove unused variable.