crypto: dh - fix memory leak

In case memory resources for *base* were allocated, release them
before return.

Addresses-Coverity-ID: 1471702 ("Resource leak")
Fixes: e3fe0ae1

 ("crypto: dh - add public key verification test")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Stephan Müller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>