Fix wrapped valid-range handling in ty_find_init_error

[joshtriplett]

Rust's niche handling allows for wrapping valid ranges with end < start;
for instance, a valid range with start=43 and end=41 means a niche of
42. Most places in the compiler handle this correctly, but
ty_find_init_error assumed that lo > 0 means the type cannot contain a
zero.

Fix it to handle wrapping ranges.