re PR libstdc++/59829 (Calling vector::data() occurs undefined behavior when the...

re PR libstdc++/59829 (Calling vector::data() occurs undefined behavior when the vector is empty)

	PR libstdc++/59829
	* include/bits/stl_vector.h (vector::data()): Call _M_data_ptr.
	(vector::_M_data_ptr): New overloaded functions to ensure empty
	vectors do not dereference the pointer.
	* testsuite/23_containers/vector/59829.cc: New.
	* testsuite/23_containers/vector/requirements/dr438/assign_neg.cc:
	Adjust dg-error line number.
	* testsuite/23_containers/vector/requirements/dr438/
	constructor_1_neg.cc: Likewise.
	* testsuite/23_containers/vector/requirements/dr438/
	constructor_2_neg.cc: Likewise.
	* testsuite/23_containers/vector/requirements/dr438/insert_neg.cc:
	Likewise.

From-SVN: r207241