Fix an out of bounds array access.

We were looking at Magic[5] without checking Length. Since this path would not
return unless Length >= 18 anyway, just move the >= 18 check up.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183666 91177308-0d34-0410-b5e6-96231b3b80d8