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/buffer/buffer.c

@@ -71,7 +71,7 @@ BUF_MEM *BUF_MEM_new(void)
 {
     BUF_MEM *ret;
 
-    ret = OPENSSL_malloc(sizeof(BUF_MEM));
+    ret = OPENSSL_malloc(sizeof(*ret));
     if (ret == NULL) {
         BUFerr(BUF_F_BUF_MEM_NEW, ERR_R_MALLOC_FAILURE);
         return (NULL);