If an engine comes up explicitely, it must also come down explicitely

In apps/apps.c, one can set up an engine with setup_engine().
However, we freed the structural reference immediately, which means
that for engines that don't already have a structural reference
somewhere else (because it's a built in engine), we end up returning
an invalid reference.

Instead, the function release_engine() is added, and called at the end
of the routines that call setup_engine().

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1643)

apps/spkac.c

diff --git a/apps/spkac.c b/apps/spkac.c
index 70eeafc..871b4f0 100644
--- a/apps/spkac.c
+++ b/apps/spkac.c
@@ -187,6 +187,7 @@
     NETSCAPE_SPKI_free(spki);
     BIO_free_all(out);
     EVP_PKEY_free(pkey);
+    release_engine(e);
     OPENSSL_free(passin);
     return (ret);
 }