ps/output.c: Always null-terminate outbuf in show_one_proc().

ps/output.c: Always null-terminate outbuf in show_one_proc().

Before "strlen(outbuf)", if one of the pr_*() functions forgot to do it.
This prevents an out-of-bounds read in strlen(), and an out-of-bounds
write in "outbuf[sz] = '\n'". Another solution would be to replace
strlen() with strnlen(), but this is not used anywhere else in the
code-base and may not exist in all libc's.