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/comp/comp_lib.c

@@ -8,7 +8,7 @@ COMP_CTX *COMP_CTX_new(COMP_METHOD *meth)
 {
     COMP_CTX *ret;
 
-    if ((ret = OPENSSL_malloc(sizeof(COMP_CTX))) == NULL) {
+    if ((ret = OPENSSL_malloc(sizeof(*ret))) == NULL) {
         /* ZZZZZZZZZZZZZZZZ */
         return (NULL);
     }