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>

crypto/objects/obj_xref.c

@@ -154,7 +154,7 @@ int OBJ_add_sigid(int signid, int dig_id, int pkey_id)
         sigx_app = sk_nid_triple_new(sigx_cmp);
     if (!sigx_app)
         return 0;
-    ntr = OPENSSL_malloc(sizeof(int) * 3);
+    ntr = OPENSSL_malloc(sizeof(*ntr));
     if (!ntr)
         return 0;
     ntr->sign_id = signid;