Define META_FREE in a way that doesn't cause -Wempty-body warnings.

Define META_FREE in a way that doesn't cause -Wempty-body warnings.

That get rids of the only -Wempty-body warning when compiling postgres
with gcc 4.8/9. As 6550b901f shows, it's useful to be able to use that
option routinely.

Without asserts there's many more warnings, but that's food for
another commit.