Avoid unnecessary memory usage.
 [rsc] --rw-rw-r-- M 771144 rsc sys 10919 Feb 11 09:15 sys/src/cmd/aux/disksim.c
	/n/sourcesdump/2006/0211/plan9/sys/src/cmd/aux/disksim.c:246,252 - 
	/n/sourcesdump/2006/0212/plan9/sys/src/cmd/aux/disksim.c:246,252
	  	uint i0, i1, i2;
	  	vlong oaddr;
	  
	- 	if(fd)
	+ 	if(fd >= 0)
	  		alloc = 1;
	  
	  	addr >>= LOGBLKSZ;