Good hygiene with size_t output argument.

Though the callers check the function return value and ignore the
size_t output argument on failure, it is still often not ideal to
store (-1) in size_t on error.  That might signal an unduly large
buffer.  Instead set the size_t to 0, to indicate no space.

Reviewed-by: Richard Levitte <levitte@openssl.org>

ssl/s3_cbc.c

@@ -278,7 +278,7 @@ int ssl3_cbc_digest_record(const EVP_MD_CTX *ctx,
          */
         OPENSSL_assert(0);
         if (md_out_size)
-            *md_out_size = -1;
+            *md_out_size = 0;
         return 0;
     }