libstdc++: Avoid double-deref of __first in ranges::minmax [PR104858]

	PR libstdc++/104858

libstdc++-v3/ChangeLog:

	* include/bits/ranges_algo.h (__minmax_fn): Avoid dereferencing
	__first twice at the start.
	* testsuite/25_algorithms/minmax/constrained.cc (test06): New test.