Fix memory leak of yaml_event_t

Fixes a number of these:
```
==7600== 112 bytes in 7 blocks are definitely lost in loss record 14 of 20
==7600==    at 0x4C2CD7B: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==7600==    by 0x4E35B5F: ??? (in /usr/lib/x86_64-linux-gnu/libyaml-0.so.2.0.2)
==7600==    by 0x4E410B4: yaml_parser_fetch_more_tokens (in /usr/lib/x86_64-linux-gnu/libyaml-0.so.2.0.2)
==7600==    by 0x4E459CA: ??? (in /usr/lib/x86_64-linux-gnu/libyaml-0.so.2.0.2)
==7600==    by 0x432500: yaml_Parser__YAMLParser_parseEvent_withEvent_impl (Parser.ooc:40)
==7600==    by 0x4325E2: yaml_Parser__YAMLParser_parseEvent_withEvent (Parser.ooc:85)
==7600==    by 0x432615: yaml_Parser__YAMLParser_parseAll_impl (Parser.ooc:53)
==7600==    by 0x4326F2: yaml_Parser__YAMLParser_parseAll (Parser.ooc:93)
==7600==    by 0x4326A6: yaml_Parser__YAMLParser_parseDocument_impl (Parser.ooc:60)
==7600==    by 0x4327EA: yaml_Parser__YAMLParser_parseDocument (Parser.ooc:97)
==7600==    by 0x431704: main (convert.ooc:8)
```