Fix some nullptr dereferences
Turns out we were getting away with dereferencing
`nullptr` in a few cases as `walk` would use
`nullptr` to indicate that the walk should be
stopped, and luckily Clang didn't optimize it to
something broken.
This commit is fairly defensive and sprinkles
some null checks for calls to `walk` directly
on a body of a function or top-level code decl.
