We need to distinguish when the option is not set in domain section and when
it is is explicitly disabled. Now if it is not set, domain->gssapi_services
is NULL and we'll use value from the pam section.

Without this change, the value in the pam section is ignored.

```
1. Defect type: RESOURCE_LEAK
7. sssd-2.4.0/src/sss_client/pam_sss_gss.c:556: leaked_storage: Variable "username" going out of scope leaks the storage it points to.
Expand
2. Defect type: RESOURCE_LEAK
3. sssd-2.4.0/src/sss_client/pam_sss_gss.c:321: leaked_storage: Variable "reply" going out of scope leaks the storage it points to.
Expand
3. Defect type: RESOURCE_LEAK
7. sssd-2.4.0/src/sss_client/pam_sss_gss.c:260: leaked_storage: Variable "username" going out of scope leaks the storage it points to.
Expand
4. Defect type: RESOURCE_LEAK
6. sssd-2.4.0/src/sss_client/pam_sss_gss.c:260: leaked_storage: Variable "upn" going out of scope leaks the storage it points to.
Expand
5. Defect type: RESOURCE_LEAK
7. sssd-2.4.0/src/sss_client/pam_sss_gss.c:260: leaked_storage: Variable "target" going out of scope leaks the storage it points to.
Expand
6. Defect type: RESOURCE_LEAK
7. sssd-2.4.0/src/sss_client/pam_sss_gss.c:260: leaked_storage: Variable "domain" going out of scope leaks the storage it points to.

1. Defect type: CLANG_WARNING
1. sssd-2.4.0/src/sss_client/pam_sss_gss.c:260:16: warning[unix.Malloc]: Potential leak of memory pointed to by 'username'
Expand
2. Defect type: CLANG_WARNING
1. sssd-2.4.0/src/sss_client/pam_sss_gss.c:260:16: warning[unix.Malloc]: Potential leak of memory pointed to by 'upn'
Expand
3. Defect type: CLANG_WARNING
1. sssd-2.4.0/src/sss_client/pam_sss_gss.c:260:16: warning[unix.Malloc]: Potential leak of memory pointed to by 'target'
Expand
4. Defect type: CLANG_WARNING
1. sssd-2.4.0/src/sss_client/pam_sss_gss.c:260:16: warning[unix.Malloc]: Potential leak of memory pointed to by 'domain'
```

Also fix compilation warning
```
../src/sss_client/pam_sss_gss.c:339:5: warning: ‘reply’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  339 |     free(reply);
      |     ^~~~~~~~~~~
../src/sss_client/pam_sss_gss.c:328:14: note: ‘reply’ was declared here
  328 |     uint8_t *reply;
      |              ^~~~~
../src/sss_client/pam_sss_gss.c:270:11: warning: ‘reply_len’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  270 |     upn = malloc(reply_len * sizeof(char));
      |           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/sss_client/pam_sss_gss.c:327:12: note: ‘reply_len’ was declared here
  327 |     size_t reply_len;
      |            ^~~~~~~~~
```