Replace memset with OPENSSL_clear_free()

BUF_MEM_free() attempts to cleanse memory using memset immediately prior
to a free. This is at risk of being optimised away by the compiler, so
replace with a call to OPENSSL_clear_free() instead.

With thanks to the Open Crypto Audit Project for reporting this issue.
Reviewed-by: NStephen Henson <steve@openssl.org>

crypto/buffer/buffer.c

@@ -88,8 +88,7 @@ void BUF_MEM_free(BUF_MEM *a)
         return;
 
     if (a->data != NULL) {
-        memset(a->data, 0, (unsigned int)a->max);
-        OPENSSL_free(a->data);
+        OPENSSL_clear_free(a->data, a->max);
     }
     OPENSSL_free(a);
 }