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ip/ethermedium: drop short packets instead of producing negative size blocks

on usb ethernet, it can happen that we read truncated packets smaller
than the ethernet header size. this produces a warning in pullupblock()
later like: "pullup negative length packet, called from 0xf0199e46"

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

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

@@ -354,11 +354,12 @@

 			nexterror();

 		}

 		ifc->in++;

-		bp->rp += ifc->m->hsize;

-		if(ifc->lifc == nil)

+		if(ifc->lifc == nil || BLEN(bp) <= ifc->m->hsize)

 			freeb(bp);

-		else

+		else {

+			bp->rp += ifc->m->hsize;

 			ipiput4(er->f, ifc, bp);

+		}

 		runlock(ifc);

 		poperror();

 	}

@@ -393,11 +394,12 @@

 			nexterror();

 		}

 		ifc->in++;

-		bp->rp += ifc->m->hsize;

-		if(ifc->lifc == nil)

+		if(ifc->lifc == nil || BLEN(bp) <= ifc->m->hsize)

 			freeb(bp);

-		else

+		else {

+			bp->rp += ifc->m->hsize;

 			ipiput6(er->f, ifc, bp);

+		}

 		runlock(ifc);

 		poperror();

 	}

-- 

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