linux-user/signal.c: Avoid using uninitialized data in ARM sigreturn

Rephrase code used in ARM sigreturn functions to avoid using
uninitialized variables. This fixes one genuine problem ('frame'
would not be initialized if we took the error-exit path because
our stackpointer was misaligned) and one which is clang being
alarmist (frame_addr wouldn't be initialized, though this is
harmless since unlock_user_struct ignores its second argument
in these cases; however since we don't generally make use of
this not-really-documented effect it's better avoided).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1375095632-13735-3-git-send-email-peter.maydell@linaro.org
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
