Use SSIZE_MAX in trees_ostream_send_chunk() check

The overflow check should use SSIZE_MAX since the function returns the
chunk_size of size_t which in theory "could be" bigger but in practice
very unlikely.

Reported-by: Tomasz Miąsko <tomasz.miasko@gmail.com>
Signed-off-by: David Goulet <dgoulet@riseup.net>

src/trees-ostream.c

@@ -85,7 +85,7 @@ trees_ostream_send_chunk(struct trees_ostream *sstream,
 {
   ssize_t ret;
   /* Extra protection here against overflow. Maybe too agressive! */
-  assert(chunk_size < (SIZE_MAX - crypto_box_SEALBYTES));
+  assert(chunk_size < (SSIZE_MAX - crypto_box_SEALBYTES));
   size_t ciphertext_len = crypto_box_SEALBYTES + chunk_size;
   unsigned char ciphertext[ciphertext_len];