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/d1_lib.c

@@ -136,11 +136,11 @@ int dtls1_new(SSL *s)
     
     if (!ssl3_new(s))
         return (0);
-    if ((d1 = OPENSSL_malloc(sizeof *d1)) == NULL) {
+    if ((d1 = OPENSSL_malloc(sizeof(*d1))) == NULL) {
         ssl3_free(s);
         return (0);
     }
-    memset(d1, 0, sizeof *d1);
+    memset(d1, 0, sizeof(*d1));
 
     d1->buffered_messages = pqueue_new();
     d1->sent_messages = pqueue_new();