del_attributes returns null or segfault when trying to delete all keys

[dputhier]

Look at this example:

gtftk get_example -d mini_real | gtftk del_attr --reg-exp --key '.*'

[dputhier]

 chr1   ensembl_havana  gene    1001138 1014541 0   +   -1  (null) "(null)";
 chr1   havana  transcript  1001138 1014541 0   +   -1  (null) "(null)";     
 chr1   havana  exon    1001138 1001281 0   +   -1  (null) "(null)";
 chr1   havana  five_prime_utr  1001138 1001281 0   +   -1  (null) "(null)";