[devhost] Fix double-free of ProxyIostate

The double free would occur if
1) some thread invoked Cancel() to queue destruction of a ProxyIostate,
   and
2) before that packet was processed, the ProxyIostate's handler ran and
   decided to destroy itself.

Bug: ZX-4060
Change-Id: I3b9c7275c4e0cd2dc3a6c6cd2d2376c947eb627a