eina debug - if calloc for session struct failed - return NULL

makes analysers happy but really doesnt change anything - moves the
abort/crash/exit somewhere else.

found by PVS studio

src/lib/eina/eina_debug.c

@@ -437,6 +437,7 @@ static Eina_Debug_Session *
 _session_create(int fd)
 {
    Eina_Debug_Session *session = calloc(1, sizeof(*session));
+   if (!session) return NULL;
    session->dispatch_cb = eina_debug_dispatch;
    session->fd = fd;
    // start the monitor thread