Fix some memory leaks

[daniloegea]

* error.c: make sure the pointer 'line' is free'd after the last
  loop iteration. Also, free error_context before returning.

* generate.c: free the error object if an error is returned at some point.

* generate.c: free the options context allocated at the start of the main
  function.

* openvswitch.c: make sure the GString "cmds" is free'd before the
  function returns.

* parse.c: free the temporary list "values" before the function
  returns.

* parse.c: parsed_defs is set to NULL preventing
  netplan_parser_reset() to free the memory allocated for the
  hash table. Not setting it to NULL is safe because all the data is moved to
  another hash table, leaving parsed_defs with no elements. Not
  nullifying parsed_defs here allows netplan_parser_reset() to free
  it.

* types.c: in this particular case we g_strdup both key and value
  strings when inserting them in the hash table in
  parse.c:handle_generic_map(). Calling the destructor for both of
  them will make sure all the memory is free'd. When using
  free_hashtable_with_destructor in the future we'll need to make
  sure both key and value are not poiting to the same place to avoid
  a double free.

* util.c: release the glob_t allocated in find_yaml_glob().

* parse-nm.c: free "ap" when ssid can't be found (caught by clang
  build-scan).

* parse.c: filepath might be overwritten in some situations. Make
  sure the old one is free'd.

* parse.c: netdef->vxlan might be overwritten in some situations.
  Make sure the old one is free'd.

* types.c: free embedded_switch_mode when the netdef is reset.

Description

The before and after can be seen by running the generate program with valgrind

assuming you have an etc/netplan in /tmp/fakeroot:
valgrind --leak-check=full ./generate -r /tmp/fakeroot/

It can also be checked by compiling netplan with ASAN:

CFLAGS=-fsanitize=address meson setup build --prefix=/usr
meson compile -C build --verbose
and then
./generate -r /tmp/fakeroot/

Checklist

• Runs make check successfully.
• Retains 100% code coverage (make check-coverage).
• New/changed keys in YAML format are documented.
• (Optional) Adds example YAML for new feature.
• (Optional) Closes an open bug in Launchpad.

[slyon]

Thank you very much, LGTM!
I really like the systematic approach of searching for memory leaks using valgrind!

[slyon]

Unrelated, but makes sense to fix this up, while touching the file!