linux-user: Use g_try_malloc() in do_msgrcv()

linux-user: Use g_try_malloc() in do_msgrcv()

In do_msgrcv() we want to allocate a message buffer, whose size
is passed to us by the guest. That means we could legitimately
fail, so use g_try_malloc() and handle the error case, in the same
way that do_msgsnd() does.

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