[TargetParser] Check size before accessing architecture version.

Summary:
This fixes a crash when invalid -march options like `armv` are provided.

Based on a patch by Will Lovett.


Reviewers: rengolin, samparker, mcrosier

Reviewed By: samparker

Subscribers: aemerson, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D41429

llvm-svn: 321166
diff --git a/llvm/lib/Support/TargetParser.cpp b/llvm/lib/Support/TargetParser.cpp
index ea12d80..b96ca08 100644
--- a/llvm/lib/Support/TargetParser.cpp
+++ b/llvm/lib/Support/TargetParser.cpp
@@ -537,7 +537,7 @@
 }
 
 unsigned llvm::AArch64::checkArchVersion(StringRef Arch) {
-  if (Arch[0] == 'v' && std::isdigit(Arch[1]))
+  if (Arch.size() >= 2 && Arch[0] == 'v' && std::isdigit(Arch[1]))
     return (Arch[1] - 48);
   return 0;
 }
@@ -633,7 +633,7 @@
   // Only match non-marketing names
   if (offset != StringRef::npos) {
     // Must start with 'vN'.
-    if (A[0] != 'v' || !std::isdigit(A[1]))
+    if (A.size() >= 2 && (A[0] != 'v' || !std::isdigit(A[1])))
       return Error;
     // Can't have an extra 'eb'.
     if (A.find("eb") != StringRef::npos)