Although 0 is not an errno value, it should still be recognized as
corresponding to a value belonging to the generic_category().

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	PR libstdc++/102425
	* src/c++11/system_error.cc
	(system_error_category::default_error_condition): Add 0 to
	switch.
	* testsuite/19_diagnostics/error_category/102425.cc: New test.

Tested x86_64-linux. Committed to trunk.