[ubsan] Fix print_stacktrace=1:fast_unwind_on_fatal=0 to correctly fallback to fast unwinder

ubsan_GetStackTrace (from 52b75108) called by
~ScopeReport leaves top/bottom zeroes in the
`!WillUseFastUnwind(request_fast_unwind)` code path.
When BufferedStackTrace::Unwind falls back to UnwindFast,
`if (stack_top < 4096) return;` will return early, leaving just one frame in the stack trace.

Fix this by always initializing top/bottom like 261d6e05.

Reviewed By: eugenis, yln

Differential Revision: https://reviews.llvm.org/D123562