devmnt: ignore zero-length blocks.
devssl: don't generate zero-length blocks.
 [rsc] --rw-rw-r-- M 323293 glenda sys 21549 Nov  4 07:39 sys/src/9/port/devmnt.c
	/n/sourcesdump/2005/1104/plan9/sys/src/9/port/devmnt.c:850,856 - 
	/n/sourcesdump/2005/1105/plan9/sys/src/9/port/devmnt.c:850,856
	  		b = devtab[m->c->type]->bread(m->c, m->msize, 0);
	  		if(b == nil)
	  			return -1;
	- 		if(BLEN(b) == 0){
	+ 		if(blocklen(b) == 0){
	  			freeblist(b);
	  			return -1;
	  		}
 [rsc] --rw-rw-r-- M 323293 glenda sys 26133 Nov  4 07:39 sys/src/9/port/devssl.c
	/n/sourcesdump/2005/1104/plan9/sys/src/9/port/devssl.c:618,624 - 
	/n/sourcesdump/2005/1105/plan9/sys/src/9/port/devssl.c:618,625
	  			if(b == nil)
	  				error("ssl message too short (digesting)");
	  			checkdigestb(s, b);
	- 			b->rp += s->diglen;
	+ 			pullblock(&b, s->diglen);
	+ 			len -= s->diglen;
	  			break;
	  		case Sdigenc:
	  			b = decryptb(s, b);
	/n/sourcesdump/2005/1104/plan9/sys/src/9/port/devssl.c:626,632 - 
	/n/sourcesdump/2005/1105/plan9/sys/src/9/port/devssl.c:627,633
	  			if(b == nil)
	  				error("ssl message too short (dig+enc)");
	  			checkdigestb(s, b);
	- 			b->rp += s->diglen;
	+ 			pullblock(&b, s->diglen);
	  			len -= s->diglen;
	  			break;
	  		}