Fix heap buffer overflow in function yaml_emitter_emit_flow_sequence_…

[ziyangc97]

…item, releated issue:#258

[ziyangc97]

In this PR, I not sure what to do when STACK_EMPTY check is failed. Are we going to print error log and return to end the process, or do we set indent to 0 and continue the process ?

[perlpunk]

Do you know how the problem can be reproduced?

[perlpunk]

As mentioned in #258, I think this shouldn't even happen, and something is wrong already before. The state machine should not get into this position. So I think this wouldn't fix it.

[perlpunk]

I think #290 is a more general fix for this

[perlpunk]

Closing this as it would only be a partial fix (same problem in yaml_emitter_emit_flow_mapping_key), and actually this code shouldn't be called anyway as it only happens when calling yaml_emitter_close after a failing yaml_emitter_dump, see comments in #258 (comment)