Fix PKCS7 memory leak

Commit f0e0fd51 was a bit over-zealous in removing a call to
X509_STORE_CTX_cleanup(). The call in question was in a loop and was
required to cleanup resources used on each iteration of the loop. Removing
this resulted in a memory leak.
Reviewed-by: NRichard Levitte <levitte@openssl.org>

crypto/pkcs7/pk7_smime.c

@@ -328,6 +328,7 @@ int PKCS7_verify(PKCS7 *p7, STACK_OF(X509) *certs, X509_STORE *store,
             i = X509_verify_cert(cert_ctx);
             if (i <= 0)
                 j = X509_STORE_CTX_get_error(cert_ctx);
+            X509_STORE_CTX_cleanup(cert_ctx);
             if (i <= 0) {
                 PKCS7err(PKCS7_F_PKCS7_VERIFY,
                          PKCS7_R_CERTIFICATE_VERIFY_ERROR);