zkVM args and env length can overflow

[kevin-valerio]

A large byte length can overflow to zero and still be used as the slice length.

That's targeting riscv32im-risc0-zkvm-elf where the implementations of std::env::args_os() and std::env::var_os() trust the host length. The code can allocate zero words and then build a huge
slice from that pointer.

Ideally we should panic instead of overflowing to avoid any huge allocation

rust/library/std/src/sys/env/zkvm.rs

Lines 18 to 19 in 98594f4

	let nwords = (nbytes + WORD_SIZE - 1) / WORD_SIZE;
	let words = unsafe { abi::sys_alloc_words(nwords) };

rust/library/std/src/sys/args/zkvm.rs

Line 19 in 98594f4

let arg_len_words = (arg_len + WORD_SIZE - 1) / WORD_SIZE;