Use common formatting for backtraces (#6977)

This makes debug_print_backtrace() use the same formatting as exception
backtraces. The only difference is that the final #{main} is omitted,

Use common formatting for backtraces (#6977)

This makes debug_print_backtrace() use the same formatting as exception
backtraces. The only difference is that the final #{main} is omitted,
because it wouldn't make sense for limited backtraces, and wasn't there
previously either.

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Handle debug_print_backtrace() from main script

If there is nothing to print, the smart_str will hold a nullptr.

Fixes oss-fuzz #33334.