ICE with codegen-units=0

[tomaka]

Invoking Rustc with rustc -C codegen-units=0 foo.rs triggers an ICE:

error: internal compiler error: unexpected panic
note: the compiler unexpectedly panicked. this is a bug.
note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/m
aster/CONTRIBUTING.md#bug-reports
note: run with `RUST_BACKTRACE=1` for a backtrace
thread 'rustc' panicked at 'index out of bounds: the len is 0 but the index is 0
', ../src/libcollections\vec.rs:1179
note: Run with `RUST_BACKTRACE=1` for a backtrace.

Probably easy to fix.

[Manishearth]

The actual ICE is from SharedCrateContext assuming that local_ccxs contains something. But it should always contain something, best to catch this early when constructing options.