debug: Avoid underflow when scanning for landing pad bytes

[bnjbvr]

I don't really know what I'm doing here, but bear with me... This landing pad bytes scanning assumes at least 2 bytes in the given buffer, but in a local test case I was trying against, the buffer was empty, so 0 bytes, so the subtract would underflow. Just adding this guard seems to make it work-ish -- I don't get full variable inspection power, but I expect that this is unrelated to my change.

[alexcrichton]

Do you know if it would be possible to add a test for this?

[bnjbvr]

Unfortunately, it's a very very large module, hard to break up. If you've got a tool or a magic command line that can take the wat and automatically reduce it, I'm willing to try it. Unfortunately I don't know enough about this code so as to be able to craft a test case from scratch.

[alexcrichton]

Ah ok, seems fine then!

[bjorn3]

You could try creduce on the .wat file. Creduce works relatively well on anything that even remotely looks like C. Wat having paired parens should be C like enough for it to work.