fix against corrupted output in parallel logging

In 8256076722 it was attempted to fix
a bug previously introduced by logging related changes.  The problem
is that a va_list can be corrupted after it has been used once, so
we need to va_copy before each successive use.

And if we copy it, we also need to use the copy, and not the original ;)

src/logging.c

@@ -261,7 +261,7 @@ static void _logp(int subsys, int level, char *file, int line,
 		 * in undefined state. Since _output uses vsnprintf and it may
 		 * be called several times, we have to pass a copy of ap. */
 		va_copy(bp, ap);
-		_output(tar, subsys, level, file, line, cont, format, ap);
+		_output(tar, subsys, level, file, line, cont, format, bp);
 		va_end(bp);
 	}
 }