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gre: check nil for pullupblock()

--- a/sys/src/9/ip/gre.c

+++ b/sys/src/9/ip/gre.c

@@ -300,6 +300,8 @@

 	/* make sure the message has a GRE header */

 	bp = pullupblock(bp, GRE_IPONLY+GRE_IPPLUSGRE);

+	if(bp == nil)

+		return;

 	gre = (GREhdr *)bp->rp;

 	gre->vihl = IP_VER4;

@@ -528,11 +530,8 @@

 	 * that when the block is forwarded, devether.c puts the block into

 	 * a queue that also uses ->next.  Just do not use ->next here!

 	 */

-	if(bp->next){

-		len = blocklen(bp);

-		bp  = pullupblock(bp, len);

-		assert(BLEN(bp) == len && bp->next == nil);

-	}

+	if(bp->next != nil)

+		bp = pullupblock(bp, blocklen(bp));

 	gre = (GREhdr *)bp->rp;

 	if(BLEN(bp) < sizeof(GREhdr) || gre->proto != IP_GREPROTO){