posix_cpu_timer: check return on cpu_timer_sample_group

If error, don't trust "now" time that it should be setting

kernel/time/posix-cpu-timers.c:1269:13: warning: 'now' may be used uninitialized in this function [-Wmaybe-uninitialized]

Change-Id: I679d99c335494bae50fd926663fad37aedb1487a
Signed-off-by: Chris Fries <cfries@google.com>