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/fileio.c

@@ -6755,7 +6755,8 @@ static bool apply_autocmds_group(event_T event, char_u *fname, char_u *fname_io,
     autocmd_fname = fname_io;
   }
   if (autocmd_fname != NULL) {
-    autocmd_fname = vim_strsave(autocmd_fname);
+    // Allocate MAXPATHL for when eval_vars() resolves the fullpath.
+    autocmd_fname = vim_strnsave(autocmd_fname, MAXPATHL);
   }
   autocmd_fname_full = false;   // call FullName_save() later