cmdline file without NULL will cause Sysinfo to be unable to get the command line string

[unknowndevQwQ]

Describe the bug
Fill the content of the entire cmdline through some dirty methods and do not keep the end of the NULL character. At this time, Sysinfo will not be able to read the command line of the corresponding process

sysinfo 0.30.13 on linux

To Reproduce
This can only be reproduced if the output of env | wc -c is greater than 4096:

run git clone https://github.com/unknowndevQwQ/killmyargv && cargo test
The expectation is that there should be a long string in left, but what actually happens is that no string is obtained. And procps-ng can read cmdline normally in this case