CID 1453653: Integer handling (NEGATIVE_RETURNS) in build_establish_intro_dos_extension()

trn_cell_extension_dos_encoded_len() returns ssize_t, but trn_cell_extension_field_setlen_field() takes size_t. This looks like a bug on #30924 (moved), copying sponsor fields across.

/src/feature/hs/hs_cell.c: 532 in build_establish_intro_dos_extension()
528       /* Set the field with the encoded DoS extension. */
529       dos_ext_encoded_len = trn_cell_extension_dos_encoded_len(dos_ext);
530       /* Set length field and the field array size length. */
531       trn_cell_extension_field_set_field_len(field, dos_ext_encoded_len);
   CID 1453653:  Integer handling issues  (NEGATIVE_RETURNS)
   "dos_ext_encoded_len" is passed to a parameter that cannot be negative.
532       trn_cell_extension_field_setlen_field(field, dos_ext_encoded_len);
533       /* Encode the DoS extension into the cell extension field. */
534       field_array = trn_cell_extension_field_getarray_field(field);