rand: don't free an mis-set pointer on error

This is adding robustness to the code.  The fix to not mis-set the pointer
is in #16636.

Fixes #16631

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from #16640)

(cherry picked from commit caf569a)

providers/implementations/rands/drbg.c

@@ -459,18 +459,18 @@ int ossl_prov_drbg_instantiate(PROV_DRBG *drbg, unsigned int strength,
 
     if (!drbg->instantiate(drbg, entropy, entropylen, nonce, noncelen,
                            pers, perslen)) {
+        cleanup_entropy(drbg, entropy, entropylen);
         ERR_raise(ERR_LIB_PROV, PROV_R_ERROR_INSTANTIATING_DRBG);
         goto end;
     }
+    cleanup_entropy(drbg, entropy, entropylen);
 
     drbg->state = EVP_RAND_STATE_READY;
     drbg->generate_counter = 1;
     drbg->reseed_time = time(NULL);
     tsan_store(&drbg->reseed_counter, drbg->reseed_next_counter);
 
  end:
-    if (entropy != NULL)
-        cleanup_entropy(drbg, entropy, entropylen);
     if (nonce != NULL)
         ossl_prov_cleanup_nonce(drbg->provctx, nonce, noncelen);
     if (drbg->state == EVP_RAND_STATE_READY)