Fix issues the fuzzer found overnight

[skx]

First failure is fun:

cat
testdata/fuzz/FuzzYAL/ba0fcef6ab521763179f52a1f2ef9fed78574074e826d61f9c362bd4ca07f62e
go test fuzz v1
[]byte("(defmacro! unless(fn*()`(~!)))(unless )")

Second failure is more trivial:

• []byte("(ord 0)")
  • "not a character"