Treat invalid input as ']' to save a cmp instruction.

If we don't care about invalid input characters, I think we can save an instruction. I believe the current behavior is to skip them. Disclaimer: I haven't tested this; I can't figure out how to provide input to the program under blinkenlights (WSL2 in Windows Terminal) or qemu-system-x86_64 other than manually keying it in, which is tedious and error-prone.