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>

ssl/bio_ssl.c

@@ -101,9 +101,8 @@ BIO_METHOD *BIO_f_ssl(void)
 
 static int ssl_new(BIO *bi)
 {
-    BIO_SSL *bs;
+    BIO_SSL *bs = OPENSSL_malloc(sizeof(*bs));
 
-    bs = OPENSSL_malloc(sizeof(BIO_SSL));
     if (bs == NULL) {
         BIOerr(BIO_F_SSL_NEW, ERR_R_MALLOC_FAILURE);
         return (0);