CID 1454769: Resource leaks in build_establish_intro_dos_extension()

Caused by #31682 (moved), which was a fix to another coverity issue.

CID 1454769:  Resource leaks  (RESOURCE_LEAK)
/src/feature/hs/hs_cell.c: 533 in build_establish_intro_dos_extension()
527                                       TRUNNEL_DOS_PARAM_TYPE_INTRO2_BURST_PER_SEC,
528                                       service_config->intro_dos_burst_per_sec);
529     
530       /* Set the field with the encoded DoS extension. */
531       ret = trn_cell_extension_dos_encoded_len(dos_ext);
532       if (BUG(ret <= 0)) {
   CID 1454769:  Resource leaks  (RESOURCE_LEAK)
   Variable "field" going out of scope leaks the storage it points to.
533         return -1;
534       })