os: Fix strtok/free crash in ComputeLocalClient

Don't reuse cmd for strtok output to ensure the proper pointer is
freed afterwards.

The code incorrectly assumed the pointer returned by strtok(cmd, ":")
would always point to cmd. However, strtok(str, sep) != str if str
begins with sep. This caused an invalid-free crash when running
a program under X with a name beginning with a colon.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=104123


Signed-off-by: Tomasz Śniatowski <kailoran@gmail.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>