linux-user: fix getrusage and wait4 failures with invalid rusage struct

Implementations of system calls getrusage and wait4 have not previously
handled correctly cases when incorrect address of struct rusage is
passed.
This change makes sure return values are correctly set for these cases.

Signed-off-by: Petar Jovanovic <petar.jovanovic@imgtec.com>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
