libstdc++: Handle invalid values in std::chrono pretty printers

This avoids an IndexError exception when printing invalid chrono::month
or chrono::weekday values.

libstdc++-v3/ChangeLog:

	* python/libstdcxx/v6/printers.py (StdChronoCalendarPrinter):
	Check for out-of-range month an weekday indices.
	* testsuite/libstdc++-prettyprinters/chrono.cc: Check invalid
	month and weekday values.