Reposition assertion to respect safe initialization
Reposition assertion to respect safe initialization

Passing `this` to `assert` before object members are fully initialized
does not pass safe init check:
```
[error] -- Error: library/src/scala/runtime/stdLibPatches/Predef.scala:7:64 ------------
[error] 7 |    if !assertion then scala.runtime.Scala3RunTime.assertFailed(message)
[error]   |                                                                ^^^^^^^
[error]   |Cannot prove that the value is fully initialized. Only initialized values may be used as arguments.
```

This change fixes the above error.

Review by @liufengyun

dotty-staging/dotty,lampepfl/dotty,sjrd/dotty,lampepfl/dotty,sjrd/dotty,dotty-staging/dotty,lampepfl/dotty,sjrd/dotty,lampepfl/dotty,dotty-staging/dotty,dotty-staging/dotty,lampepfl/dotty,dotty-staging/dotty,sjrd/dotty,sjrd/dotty