Rope iterator patch reminder

[Jeremy Sawicki]

Last month I sent a patch for a bug in the rope template.  I want to 
make sure it is not forgotten:

https://gcc.gnu.org/ml/libstdc++/2018-05/msg00115.html
https://gcc.gnu.org/ml/gcc-patches/2018-05/msg01006.html

When an iterator is copied or assigned, the copy can retain pointers to 
an internal buffer in the original.  When the copy is later 
dereferenced, problems ranging from incorrect results to undefined 
behavior can occur depending on what has happened to the original in the 
meantime.  See the test in the patch for a reproducible case.

Please let me know if anything else is needed from me.

Jeremy