gnrc_pktbuf_static: allow write-protect of size 0 snips

Size 0 snips are legal packet snips (empty payload e.g.) so it doesn't
make sense to issue an error in the write-protection in that case.

API documentation doesn't mention it either and the tests still pass
with the check removed.

sys/net/gnrc/pktbuf_static/gnrc_pktbuf_static.c

@@ -249,7 +249,7 @@ void gnrc_pktbuf_release_error(gnrc_pktsnip_t *pkt, uint32_t err)
 gnrc_pktsnip_t *gnrc_pktbuf_start_write(gnrc_pktsnip_t *pkt)
 {
     mutex_lock(&_mutex);
-    if ((pkt == NULL) || (pkt->size == 0)) {
+    if (pkt == NULL) {
         mutex_unlock(&_mutex);
         return NULL;
     }