wm stackorder panics

The TkWmStackorderToplevel function in unix/tkUnixWm.c
seems to incorrectly calculate the number of toplevels
when they are displayed on different displays. This
results in the message "num matched toplevel windows
does not equal num children" and then wish aborts.

Script to reproduce the situation:

# wish -display :0
% toplevel .x -screen :1
.x
% wm stackorder .
num matched toplevel windows does not equal num children
Aborted

In a conversation on the Tcler's chat Joe English
remarked the following about this:
The point where it's panicking looks to me like it's
not really an error condition at all (let alone a fatal
one) -- More like "I'm making an assumption about what
the state of the system should be, and crashing hard if
that assumption isn't true". But in this case the
assumption in question (a) doesn't always hold and
probably (b) doesn't even need to hold for the code to
work.