Don't dereference nil.
 [rsc] --rw-rw-r-- M 1589387 glenda sys 13379 Oct 16 11:08 sys/src/cmd/bc.y
	/n/sourcesdump/2005/1016/plan9/sys/src/cmd/bc.y:796,802 - 
	/n/sourcesdump/2005/1017/plan9/sys/src/cmd/bc.y:796,803
	  		ln = 0;
	  		goto loop;
	  	}
	- 	Bterm(in);
	+ 	if(in)
	+ 		Bterm(in);
	  	if((in = Bopen(sargv[ifile], OREAD)) != 0){
	  		ln = 0;
	  		ss = sargv[ifile];
 [sys] --rwxrwxr-x M 1589387 rsc sys 81774 Oct 16 23:09 386/bin/bc
	/sys/src/cmd/bc.y:getch