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

test/functional/provider/ruby_spec.lua

@@ -1,16 +1,18 @@
 local helpers = require('test.functional.helpers')(after_each)
 
-local eq = helpers.eq
-local feed = helpers.feed
 local clear = helpers.clear
-local funcs = helpers.funcs
-local meths = helpers.meths
-local insert = helpers.insert
-local expect = helpers.expect
 local command = helpers.command
-local write_file = helpers.write_file
 local curbufmeths = helpers.curbufmeths
+local eq = helpers.eq
+local eval = helpers.eval
+local expect = helpers.expect
+local feed = helpers.feed
+local feed_command = helpers.feed_command
+local funcs = helpers.funcs
+local insert = helpers.insert
+local meths = helpers.meths
 local missing_provider = helpers.missing_provider
+local write_file = helpers.write_file
 
 do
   clear()
@@ -90,3 +92,11 @@ describe(':rubydo command', function()
     eq(false, curbufmeths.get_option('modified'))
   end)
 end)
+
+describe('ruby provider', function()
+  it('RPC call to expand("<afile>") during BufDelete #5245 #5617', function()
+    command([=[autocmd BufDelete * ruby VIM::evaluate('expand("<afile>")')]=])
+    feed_command('help help')
+    eq(2, eval('1+1'))  -- Still alive?
+  end)
+end)