Fix DwarfExpression::AddMachineRegExpression so it doesn't read past the

Fix DwarfExpression::AddMachineRegExpression so it doesn't read past the
end of an expression that ends with DW_OP_plus.
Caught by the ASAN build bots.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231260 91177308-0d34-0410-b5e6-96231b3b80d8