linux-user/signal.c: Correct error path for AArch64 do_rt_sigreturn

The error path in AArch64 do_rt_sigreturn() which fails before
attempting lock_user_struct() was doing an unlock_user_struct()
on an uninitialized variable. Initialize frame to NULL so we
can use the same error-exit path in all cases (unlock of NULL
is permitted and does nothing).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
