extract_tag_der_recursive: fix compilation errors

Fix the following errors introduced by extract_tag_der_recursive usage:

decoding.c: In function 'extract_tag_der_recursive':
decoding.c:560:5: error: 'ris' may be used uninitialized in this function [-Werror=maybe-uninitialized]

decoding.c: In function 'asn1_der_decoding_startEnd':
decoding.c:2192:40: error: unused variable 'p3' [-Werror=unused-variable]

Signed-off-by: Nikos Mavrogiannopoulos <nmav@redhat.com>
asn1_write_value: allow SET_OF elements deletion

SET_OF elements could just be deleted as SEQUENCE_OF ones.

Signed-off-by: Nikos Mavrogiannopoulos <nmav@redhat.com>
cleaned up code
asn1_write_value: fix segfault when deleting an unauthorized element

Segfaults can occur, since value is dereferenced later, ex:

 352       if ((isdigit (value[0])) || (value[0] == '-')

Signed-off-by: Nikos Mavrogiannopoulos <nmav@redhat.com>
doc update