Fix double-free.
 [rsc] --rw-rw-r-- M 374841 glenda sys 16904 Nov 28 07:26 sys/src/lib9p/srv.c
	/n/sourcesdump/2005/1128/plan9/sys/src/lib9p/srv.c:806,811 - 
	/n/sourcesdump/2005/1129/plan9/sys/src/lib9p/srv.c:806,813
	  	for(i=0; i<r->nflush; i++)
	  		respond(r->flush[i], nil);
	  	free(r->flush);
	+ 	r->flush = nil;
	+ 	r->nflush = 0;
	  
	  	if(r->pool)
	  		closereq(r);