memset, memcpy, sizeof consistency fixes

Just as with the OPENSSL_malloc calls, consistently use sizeof(*ptr)
for memset and memcpy.  Remove needless casts for those functions.
For memset, replace alternative forms of zero with 0.

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

crypto/stack/stack.c

@@ -296,7 +296,7 @@ void sk_zero(_STACK *st)
         return;
     if (st->num <= 0)
         return;
-    memset((char *)st->data, 0, sizeof(*st->data) * st->num);
+    memset(st->data, 0, sizeof(*st->data) * st->num);
     st->num = 0;
 }