[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346979 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed