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/engine/eng_openssl.c

@@ -425,7 +425,7 @@ typedef struct {
 static int ossl_hmac_init(EVP_PKEY_CTX *ctx)
 {
     OSSL_HMAC_PKEY_CTX *hctx;
-    hctx = OPENSSL_malloc(sizeof(OSSL_HMAC_PKEY_CTX));
+    hctx = OPENSSL_malloc(sizeof(*hctx));
     if (!hctx)
         return 0;
     hctx->md = NULL;