json: use secure un{base64,hex}mem for sensitive variants

While tracing a LUKS code path in homework, I've noticed that we don't
erase buffers when doing unbase64 or unhex on JSON variants, even if the
variant is marked as sensitive.

(cherry picked from commit 80313c5)
(cherry picked from commit cce7df4)
(cherry picked from commit 2dee004)

src/shared/json.c

@@ -4874,14 +4874,14 @@ int json_variant_unbase64(JsonVariant *v, void **ret, size_t *ret_size) {
         if (!json_variant_is_string(v))
                 return -EINVAL;
 
-        return unbase64mem(json_variant_string(v), SIZE_MAX, ret, ret_size);
+        return unbase64mem_full(json_variant_string(v), SIZE_MAX, /* secure= */ json_variant_is_sensitive(v), ret, ret_size);
 }
 
 int json_variant_unhex(JsonVariant *v, void **ret, size_t *ret_size) {
         if (!json_variant_is_string(v))
                 return -EINVAL;
 
-        return unhexmem(json_variant_string(v), SIZE_MAX, ret, ret_size);
+        return unhexmem_full(json_variant_string(v), SIZE_MAX, /* secure= */ json_variant_is_sensitive(v), ret, ret_size);
 }
 
 static const char* const json_variant_type_table[_JSON_VARIANT_TYPE_MAX] = {