Trying to write to a file in a folder that doesn't exist breaks lalpulsar_spec_avg_long

If the user asks the output base name to point to a folder that doesn't exist, lalpulsar_spec_avg_long crashes with a seg fault rather than exiting with an error. Probably the fix includes a check on whether the return value from calls to fopen() is NULL or not.

It is likely this is a problem with lalpulsar_spec_avg and lalpulsar_spec_coherence as well. These codes should be checked.