avoid overflow on functionality to print backtrace of safepoint straggler (#57579)
In the line of C code:
```C
const int64_t timeout = jl_options.timeout_for_safepoint_straggler_s * 1000000000;
```
`jl_options.timeout_for_safepoint_straggler_s` is an `int16_t` and
`1000000000` is an `int32_t`.
The result of `jl_options.timeout_for_safepoint_straggler_s *
1000000000` will be an `int32_t` which may not be large enough to hold
the value of `jl_options.timeout_for_safepoint_straggler_s` after
converting to nanoseconds, leading to overflow.