This clarifies that it is legal to return an invalid pointer from a
function, provided that such pointer is not dereferenced.

This matches current status quo of the language. Any change to this
should be a proposal that argues for different semantics.

It is also legal in C to return a pointer to a local. The C backend
lowers such thing directly, so the corresponding warning in C must be
disabled (`-Wno-return-stack-address`).