Fix false positive static analysis issue #2

Fix false positive static analysis issue #2

Infer complains:
```
test_digest.c:662: error: Dead Store
  The value written to `&p` is never used.
  660.     if (t->truncate) {
  661.         outsize = t->truncate;
  662. 	params[p++] = OSSL_PARAM_construct_size_t("size", &outsize);
               ^
  663.     }
  664.     else
```

Altough the complain is false positive, fixing it will allow for future
use of the tools in automated tests.