libprocessgroup: Add argument checks in KillProcessGroup()

Apparently there is Java code that calls KillProcessGroup() with an
invalid initialPid argument. Hence this CL that makes KillProcessGroup()
fail early if one of its arguments is invalid.

Change-Id: I42f98eed139d9d0950428d04180e4613ba74b4e6
Signed-off-by: Bart Van Assche <bvanassche@google.com>