queue: fix QTAILQ_FOREACH_REVERSE_SAFE

queue: fix QTAILQ_FOREACH_REVERSE_SAFE

The iteration was stopping as soon as prev_var was set to NULL, and
therefore it skipped the first element.  Fortunately, or unfortunately,
we have only one use of QTAILQ_FOREACH_REVERSE_SAFE.  Thus this only
showed up as incorrect register preferences on the very first translation
block that was compiled.

Reported-by: Thomas Huth <[email protected]>
Reviewed-by: Emilio G. Cota <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>