coap_session.c: minimize buffer space for coap_session_str()

The buffer szSession for storing a textual representation of the
session structure has been trimmed down to 2 * (INET6_ADDRSTRLEN + 8)
+ 24, allowing two IPv6 address references, port numbers, some
structuring text and about eight characters for the interface index.

Background: coap_print_addr() calls inet_ntop() for address output
which on RIOT is mapped to ipv6_addr_to_str(). As the length parameter
of ipv6_addr_to_str() is a uint8_t, only the lower eight bits of
szSession's size are effectively used. The previous value of 256 then
always lead no ENOSPC because 256 & 0xff == 0.

src/coap_session.c

@@ -1187,7 +1187,7 @@ coap_session_get_by_peer(coap_context_t *ctx,
 }
 
 const char *coap_session_str(const coap_session_t *session) {
-  static char szSession[256];
+  static char szSession[2 * (INET6_ADDRSTRLEN + 8) + 24];
   char *p = szSession, *end = szSession + sizeof(szSession);
   if (coap_print_addr(&session->addr_info.local,
                       (unsigned char*)p, end - p) > 0)