[MC, RISCV] Fixed StringRef Assertion `Index < Length && "Invalid ind…

…ex!"'

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

lib/MC/MCParser/AsmParser.cpp

@@ -1842,7 +1842,7 @@ bool AsmParser::parseStatement(ParseStatementInfo &Info,
   // Otherwise, we have a normal instruction or directive.
 
   // Directives start with "."
-  if (IDVal[0] == '.' && IDVal != ".") {
+  if (IDVal.startswith(".") && IDVal != ".") {
     // There are several entities interested in parsing directives:
     //
     // 1. The target-specific assembly parser. Some directives are target

test/Assembler/empty-string.s

@@ -0,0 +1,3 @@
+// REQUIRES: riscv-registered-target
+// RUN: not llvm-mc -triple riscv32-unknown-linux-gnu < %s 2>&1 | FileCheck %s
+"" # CHECK: error: unrecognized instruction mnemonic