[XRay][compiler-rt] Fix oob memory access in FDR BufferQueue iterator (#90940)

Before this change, the FDR BufferQueue iterator could access oob memory
due to checks of the form `!Buffers[Offset].Used && Offset != Max`. This
allows access to `Buffers[Max]`, which is past the end of the `Buffers`
array. This can lead to crashes when that memory is not mapped. Fix this
by testing `Offset != Max` first.