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/ssl_lib.c

@@ -272,7 +272,7 @@ SSL *SSL_new(SSL_CTX *ctx)
         return (NULL);
     }
 
-    s = OPENSSL_malloc(sizeof(SSL));
+    s = OPENSSL_malloc(sizeof(*s));
     if (s == NULL)
         goto err;
     memset(s, 0, sizeof(SSL));
@@ -1844,7 +1844,7 @@ SSL_CTX *SSL_CTX_new(const SSL_METHOD *meth)
         SSLerr(SSL_F_SSL_CTX_NEW, SSL_R_X509_VERIFICATION_SETUP_PROBLEMS);
         goto err;
     }
-    ret = OPENSSL_malloc(sizeof(SSL_CTX));
+    ret = OPENSSL_malloc(sizeof(*ret));
     if (ret == NULL)
         goto err;