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/dsa/dsa_lib.c

@@ -117,7 +117,7 @@ DSA *DSA_new_method(ENGINE *engine)
 {
     DSA *ret;
 
-    ret = OPENSSL_malloc(sizeof(DSA));
+    ret = OPENSSL_malloc(sizeof(*ret));
     if (ret == NULL) {
         DSAerr(DSA_F_DSA_NEW_METHOD, ERR_R_MALLOC_FAILURE);
         return (NULL);