xz: Move suffix check after stdout mode is detected.

xz: Move suffix check after stdout mode is detected.

This fixes a bug introduced in cc5aa9ab138beeecaee5a1e81197591893ee9ca0
when the suffix check was initially moved. This caused a situation that
previously worked:

    echo foo | xz -Fraw --lzma1 | wc -c

to fail because the old code knew that this would write to standard out
so a suffix was not needed.