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.

(cherry picked from commit cbd0fe2)

Author: jwakely

libstdc++-v3/include/std/stacktrace

@@ -487,7 +487,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       at(size_type __n) const
       {
 	if (__n >= size())
-	  __throw_out_of_range("basic_stack_trace::at: bad frame number");
+	  __throw_out_of_range("basic_stacktrace::at: bad frame number");
 	return begin()[__n];
       }
 

libstdc++-v3/testsuite/19_diagnostics/stacktrace/hash.cc

@@ -12,9 +12,10 @@ test_hash()
   using Alloc = __gnu_test::uneq_allocator<std::stacktrace_entry>;
   using S = std::basic_stacktrace<Alloc>;
   S s;
+  S cur = S::current();
   std::size_t h = std::hash<S>()(s);
-  std::size_t h2 = std::hash<S>()(S::current());
-  VERIFY( h != h2 );
+  std::size_t h2 = std::hash<S>()(cur);
+  VERIFY( cur.empty() == (h == h2) );
 }
 
 int main()