a better fix for the XML parsing issue when db_nmap -h

add an fd.rewind so fd.eof? can be called to prevent parsing an empty
file such as when db_nmap -h is called, which produces a trace if using
nokogiri, but not the old parser.  I believe the cost is very low since
all you are doing is redirecting the current file pointer to the file's
beginning, but I have not done any testing