don't crash if the icmp request came in fragmented.  this will
happen because ipoput4 will assert if bp->next for a less-than-medialen
packet.  example of how to trigger
ping -s 1500 target;. in my case i was getting two frags of 1492 and 8 bytes.


Notes:
Thu Mar 29 14:29:53 EDT 2012 geoff
    probably icmp6.c needs the same fix.


Reference: /n/sources/patch/applied/icmp-frag
Date: Tue Mar 13 06:25:59 CET 2012
Signed-off-by: quanstro@quanstro.net
Reviewed-by: geoff

--- /sys/src/9/ip/icmp.c	Tue Mar 13 06:23:45 2012
+++ /sys/src/9/ip/icmp.c	Tue Mar 13 06:23:42 2012
@@ -366,7 +366,7 @@
 	case EchoRequest:
 		if (iplen < n)
 			bp = trimblock(bp, 0, iplen);
-		r = mkechoreply(bp);
+		r = mkechoreply(concatblock(bp));
 		ipriv->out[EchoReply]++;
 		ipoput4(icmp->f, r, 0, MAXTTL, DFLTTOS, nil);
 		break;