Fix: check ogg_sync_buffer() pointer before writing to it

ogg_sync_buffer() can return NULL on several scenarios, the first one
being if ogg_sync_check() fails, but it can also happen in case of an
allocation failure, or internal bug.

src/format_ogg.c

@@ -92,6 +92,8 @@ static int send_ogg(shout_t *self, const unsigned char *data, size_t len)
     ogg_page     page;
 
     buffer = ogg_sync_buffer(&ogg_data->oy, len);
+    if (!buffer)
+        return self->error = SHOUTERR_INSANE;
     memcpy(buffer, data, len);
     ogg_sync_wrote(&ogg_data->oy, len);