Use safer sizeof variant in malloc

For a local variable:
        TYPE *p;
Allocations like this are "risky":
        p = OPENSSL_malloc(sizeof(TYPE));
if the type of p changes, and the malloc call isn't updated, you
could get memory corruption.  Instead do this:
        p = OPENSSL_malloc(sizeof(*p));
Also fixed a few memset() calls that I noticed while doing this.

Reviewed-by: Richard Levitte <levitte@openssl.org>

ssl/ssl_conf.c

diff --git a/ssl/ssl_conf.c b/ssl/ssl_conf.c
index 2d96b11..a14f564 100644
--- a/ssl/ssl_conf.c
+++ b/ssl/ssl_conf.c
@@ -609,9 +609,9 @@
 
 SSL_CONF_CTX *SSL_CONF_CTX_new(void)
 {
-    SSL_CONF_CTX *ret;
+    SSL_CONF_CTX *ret = OPENSSL_malloc(sizeof(*ret));
     size_t i;
-    ret = OPENSSL_malloc(sizeof(SSL_CONF_CTX));
+
     if (ret) {
         ret->flags = 0;
         ret->prefix = NULL;