libstdc++: Do not allocate a zero-size vector<bool> [PR 100153]

libstdc++: Do not allocate a zero-size vector<bool> [PR 100153]

The vector<bool>::shrink_to_fit() implementation will allocate new
storage even if the vector is empty. That then leads to the
end-of-storage pointer being non-null and equal to the _M_start._M_p
pointer, which means that _M_end_addr() has undefined behaviour.

The fix is to stop doing a useless zero-sized allocation in
shrink_to_fit(), so that _M_start._M_p and _M_end_of_storage are both
null after an empty vector shrinks.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	PR libstdc++/100153
	* include/bits/vector.tcc (vector<bool>::_M_shrink_to_fit()):
	When size() is zero just deallocate and reset.