Fixes a typo that causes Reserve(X) to both re-allocate the container and
change the last index (i.e. to X), which has unintended consequences for
Reserve(X) followed by Append().


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@190785 91177308-0d34-0410-b5e6-96231b3b80d8