backtrace: don't cons large lists when RCX is overwritten inside XEPs.

To present a list with the actual number of passed arguments in the
backtrace, clean-xep used the arg-count register and added missing
arguments in the form of #<unknown>, but if the register is
overwritten by other code, it could cons very large lists, exhausting
heap. Do such arg-list clean up only upon INVALID-ARG-COUNT-ERROR.
Fixes lp#1192929.