(backtrace--print-func-and-args): Fix (part of) bug#70436

(backtrace--print-func-and-args): Fix (part of) bug#70436

The source of bug#70436 is that we print a value into the buffer
and then we generate its print representation a second time to
get its length to find the bounds of the thing we just printed.
Not only it's wasteful, but it risks bugs because the two
"prints" can be inconsistent with each other.

This is not a complete fix because in the non EVALD case we
still use that same broken way.

* lisp/emacs-lisp/backtrace.el (backtrace--print-func-and-args):
Don't re-print things just to get their length.
(backtrace--print-to-string): Skip a temp-buffer indirection.