During provider dispatch, eval_call_provider() saves global
state--including pointers, such as `autocmd_fname`--into
`provider_caller_scope` which is later restored by f_rpcrequest().
But `autocmd_fname` is special-cased in eval_vars(), for performance
(see Vim patch 7.2.021; this is also the singular purpose of the
`autocmd_fname_full` global. Yay!)
If eval_vars() frees `autocmd_fname` then its provider-RPC-scoped alias
becomes a problem.
Solution: Don't free autocmd_fname in eval_vars(), just copy into it.
closes#5245closes#5617
Reference
------------------------------------------------------------------------
Vim patch 7.2.021
f6dad43c98
Problem: When executing autocommands getting the full file name may be
slow. (David Kotchan)
Solution: Postpone calling FullName_save() until autocmd_fname is used.
vim_dev discussion (2008): "Problem with CursorMoved AutoCommand when
Editing Files on a Remote WIndows Share"
https://groups.google.com/d/msg/vim_dev/kj95weZa_eE/GTgj4aq5sIgJ
