libstdc++: std::stacktrace tweaks

Fix a typo in a string literal and make the new hash.cc test gracefully
handle missing stacktrace data (see PR 112541).

libstdc++-v3/ChangeLog:

	* include/std/stacktrace (basic_stacktrace::at): Fix class name
	in exception message.
	* testsuite/19_diagnostics/stacktrace/hash.cc: Do not fail if
	current() returns a non-empty stacktrace.