Fix a possible crash in dsa_builtin_paramgen2.

Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3675)

crypto/dsa/dsa_gen.c

@@ -376,6 +376,8 @@ int dsa_builtin_paramgen2(DSA *ret, size_t L, size_t N,
     } else {
         p = BN_CTX_get(ctx);
         q = BN_CTX_get(ctx);
+        if (q == NULL)
+            goto err;
     }
 
     if (!BN_lshift(test, BN_value_one(), L - 1))