conf: Fix the problem which cause libvirtd to crash

* src/conf/domain_conf.c: Use STREQ_NULLABLE instead of STREQ,
as def->seclables[i]->model could be NULL.

src/conf/domain_conf.c

@@ -14995,7 +14995,7 @@ virDomainDiskDefGetSecurityLabelDef(virDomainDiskDefPtr def, const char *model)
         return NULL;
 
     for (i = 0; i < def->nseclabels; i++) {
-        if (STREQ(def->seclabels[i]->model, model))
+        if (STREQ_NULLABLE(def->seclabels[i]->model, model))
             return def->seclabels[i];
     }
     return NULL;