[lli] Fix crash on empty entry-function

Empty entry-function triggers the following assertion:

  llvm/lib/IR/Mangler.cpp:38: void getNameWithPrefixImpl(llvm::raw_ostream
  &, const llvm::Twine &, (anonymous namespace)::ManglerPrefixTy, const
  llvm::DataLayout &, char):

  Assertion `!Name.empty() && "getNameWithPrefix requires non-empty name"' failed.

Throw an error if entry-function is empty.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D156516