provider/RPC: apply_autocmds_group(): fix double-free

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 #5245
closes #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

src/nvim/eval.c

@@ -13828,7 +13828,7 @@ static void f_rpcrequest(typval_T *argvars, typval_T *rettv, FunPtr fptr)
     save_autocmd_fname_full = autocmd_fname_full;
     save_autocmd_bufnr = autocmd_bufnr;
     save_funccalp = save_funccal();
-    //
+
     current_SID = provider_caller_scope.SID;
     sourcing_name = provider_caller_scope.sourcing_name;
     sourcing_lnum = provider_caller_scope.sourcing_lnum;
@@ -22481,7 +22481,8 @@ typval_T eval_call_provider(char *provider, char *method, list_T *arguments)
   restore_funccal(provider_caller_scope.funccalp);
   provider_caller_scope = saved_provider_caller_scope;
   provider_call_nesting--;
-  
+  assert(provider_call_nesting >= 0);
+
   return rettv;
 }