Only set the verify callback if there's one to set!

ssl/ssl_cert.c

diff --git a/ssl/ssl_cert.c b/ssl/ssl_cert.c
index 8a53b9f..26da280 100644
--- a/ssl/ssl_cert.c
+++ b/ssl/ssl_cert.c
@@ -471,7 +471,8 @@
 
 	X509_STORE_CTX_purpose_inherit(&ctx, i, s->purpose, s->trust);
 
-	X509_STORE_CTX_set_verify_cb(&ctx, s->verify_callback);
+	if(s->verify_callback)
+		X509_STORE_CTX_set_verify_cb(&ctx, s->verify_callback);
 
 	if (s->ctx->app_verify_callback != NULL)
 		i=s->ctx->app_verify_callback(&ctx); /* should pass app_verify_arg */