gnrc_sixlowpan_iphc: check ptr != NULL in compressible check

Not only does this leave open a risk to crash the node for the check
in `_compressible()` but also is the `tmp` check below getting confused
when `ptr` is `NULL`, since `gnrc_pktbuf_start_write()` returns `NULL`
in that case.

sys/net/gnrc/network_layer/sixlowpan/iphc/gnrc_sixlowpan_iphc.c

@@ -673,7 +673,7 @@ void gnrc_sixlowpan_iphc_send(gnrc_pktsnip_t *pkt, void *ctx, unsigned page)
     dispatch = NULL;    /* use dispatch as temporary pointer for prev */
     /* determine maximum dispatch size and write protect all headers until
      * then because they will be removed */
-    while (_compressible(ptr)) {
+    while ((ptr != NULL) && _compressible(ptr)) {
         gnrc_pktsnip_t *tmp = gnrc_pktbuf_start_write(ptr);
 
         if (tmp == NULL) {