Fix SEGV in InlineAsm::ConstraintInfo::Parse.

Fixes bug 24646. Previous code was not checking if an index into a vector
was valid, resulting in a SEGV. Fixed by assuming the construct can't
be parsed when given this input.

llvm-svn: 246773
diff --git a/llvm/lib/IR/InlineAsm.cpp b/llvm/lib/IR/InlineAsm.cpp
index a181c5e..b7309b4 100644
--- a/llvm/lib/IR/InlineAsm.cpp
+++ b/llvm/lib/IR/InlineAsm.cpp
@@ -159,6 +159,8 @@
       // If Operand N already has a matching input, reject this.  An output
       // can't be constrained to the same value as multiple inputs.
       if (isMultipleAlternative) {
+        if (multipleAlternativeIndex >= ConstraintsSoFar[N].multipleAlternatives.size())
+          return true;
         InlineAsm::SubConstraintInfo &scInfo =
           ConstraintsSoFar[N].multipleAlternatives[multipleAlternativeIndex];
         if (scInfo.MatchingInput != -1)
@@ -290,4 +292,3 @@
   if (Ty->getNumParams() != NumInputs) return false;
   return true;
 }
-