libobs/util: Write 0 character when converting empty strings

Fixes log_windows_version logging garbage for build when running with
debug heap enabled (assuming osvi.szCSDVersion is an empty string)
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.