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_both.c

@@ -170,7 +170,7 @@ static hm_fragment *dtls1_hm_fragment_new(unsigned long frag_len,
     unsigned char *buf = NULL;
     unsigned char *bitmask = NULL;
 
-    frag = OPENSSL_malloc(sizeof(hm_fragment));
+    frag = OPENSSL_malloc(sizeof(*frag));
     if (frag == NULL)
         return NULL;