fix(shell): check argc in peek command

[frenchcast1234]

peek reads args[1] without checking argc, so running peek with no argument dereferences NULL inside htoi.
It doesn't fault: page 0 is identity-mapped (mmu.zig:56-64), so it silently parses garbage and dumps an arbitrary address.
Adds the same argc guard already used by memdump and poke.

[pepedinho]

thank for yout contribution to AuriOS :)