[MC, RISCV] Fixed StringRef Assertion `Index < Length && "Invalid index!"'

Summary:
Handle the case IDVal is an empty string.

This bug was uncovered by a LLVM MC Assembler Protocol Buffer
Fuzzer  for the RISC-V assembly language.

Reviewers: rnk

Reviewed By: rnk

Subscribers: rnk, niravd, pcc, peter.smith, asb, grosbach, llvm-commits, bcain, kito-cheng, shiva0217, rogfer01, PkmX

Differential Revision: https://reviews.llvm.org/D50808

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@340678 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed