Fix calculation of physical memory on linux systems

As stated in the sysinfo manpage the totalram field in the sysinfo
structure is in mem_unit sizes since Linux 2.3.23. To get the actual
memory in the system the totalram value has to be multiplied with the
mem_unit size.
Fix crash in obs_get_video_info when video_output_get_info returns NULL
Merge pull request #274 from reboot/fix_crash_on_video_init_error

Fix crash in obs_get_video_info when video_output_get_info returns NULL