shada,functests: Test how ShaDa support code reacts on errors

Some notes:
- Replaced msgpack_unpacker usage with regular xmalloc’ed buffer. Also since
  msgpack_unpack_next (as well as msgpack_unpacker_next) is not ever going to
  return MSGPACK_UNPACK_EXTRA_BYTES this condition was checked manually.
  Function that does return this status is msgpack_unpack, but it is marked as
  obsolete.
- Zero type is checked prior to main switch in shada_read_next_item because
  otherwise check would be skipped.
- Zeroing entry at the start of shada_read_next_item makes it safer.
- dedent('') does not work.
- v:oldfiles list is only replaced with bang, if it is NULL or empty.

test/functional/helpers.lua

@@ -220,9 +220,12 @@ local function execute(...)
 end
 
 -- Dedent the given text and write it to the file name.
-local function write_file(name, text)
+local function write_file(name, text, dont_dedent)
   local file = io.open(name, 'w')
-  file:write(dedent(text))
+  if not dont_dedent then
+    text = dedent(text)
+  end
+  file:write(text)
   file:flush()
   file:close()
 end