Revert "rebuild from source files when Makefile changed"

This reverts commit 10cf89453d1e5599b7191c6b22fbc7d97c5e628c, which
breaks the makefile for nmake.
Make the P6opaque null sentinel more robust.
Avoid segfault on bad outer indexing.

We'll be able to optimize this away in the future, but should not just
blindly trust the bytecode.