[WebAssembly] Fix return type of nextByte

Summary:
The old return type did not allow for correct error reporting and was
causing a compiler warning.

Reviewers: aheejin

Subscribers: dschuff, sbc100, jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D54586

llvm-svn: 346979