small fixes to ggc-zone.c

[Paolo Bonzini]

> (At least, i think i can approve this, i can never remember whether i
> have maintainership over zone or not :P)

 From http://thread.gmane.org/gmane.comp.gcc.patches/61372 (March 2004)

 > > Fine by me :)
 > > (I don't remember whether i can still approve zone collector
 > > patches.  I  think I can, but it's not clear)
 >
 > Well, you did so last December and nobody complained.

Nobody complained in that occasion as well. :-P  BTW, your approval from 
December 2003 is mentioned in 
http://gcc.gnu.org/ml/gcc-patches/2003-12/msg01097.html

Paolo