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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@246773 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed