fix invalid write in pop_cmd()

fix invalid write in pop_cmd()

The memmove() in pop_cmd() reads and writes beyond the end of argv.

This is basically harmless in the current C program; the environment
variable list immediately follows argv so all this does is unnecessarily
copy the beginning of that list.

However, this will become problematic once we start calling C functions
like fs_cmds() from Rust code. Then argv will be a Vec<String> (as
*mut *mut i8) and the memory layout will be different--in particular,
I don't think we can assume that a Vec<String> will be NULL-terminated
like argv always is--, meaning the invalid write could lead to heap
corruption.

Also, it doesn't look like full_cmd ever gets used after calling
pop_cmd() so I'm removing it here since it looks unneeded to me.

Signed-off-by: Thomas Bertschinger <tahbertschinger@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>