Fix a bug introduced in 1.11 (and also MFCd to stable AND the security branch)

Fix a bug introduced in 1.11 (and also MFCd to stable AND the security branch)
that causes a machine to panic when the kernel PPP / DEFLATE code is used.
1.11 moved a ZFREE to a point after the structural members were clobbered
by stores into a union'd structure.

This commit fixes the bug and adds a big whopping comment to make sure
the code isn't 'cleaned up' again :-)

Ian Dowse came up with the same patch independantly 68 seconds before I
did, talk about Karma!

I would also like to thank Eugene Grosbein for marathon work in tracking the
problem down by udpating his -stable based on date over and over again
to close in on the commit that caused his crashes.

PR:		kern/35969
Reviewed by:	Ian Dowse <iedowse@maths.tcd.ie>
X-MFC after:	 immediately