Don't show free blocks as leaked.
 [rsc] --rw-rw-r-- M 187561 glenda sys 5952 Sep 16 10:09 sys/src/cmd/aux/acidleak.c
	/n/sourcesdump/2005/0916/plan9/sys/src/cmd/aux/acidleak.c:271,277 - 
	/n/sourcesdump/2005/0917/plan9/sys/src/cmd/aux/acidleak.c:271,277
	  					else
	  						nhdr++;
	  				}
	- 				if(b->mark == 0)
	+ 				if(b->mark == 0 && !b->free)
	  					nleak++;
	  				nb++;
	  			}
 [sys] --rwxrwxr-x M 187561 glenda sys 69436 Sep 16 23:07 386/bin/aux/acidleak
	/sys/src/cmd/aux/acidleak.c:main