Security fix in libyaml: Limit depth of nesting by default

[hi-kumar]

Update libyaml:

• Bring over security fix in libYaml: 51843fe (PR 287)

Each nesting level increases the stack and the number of previous
starting events that the parser has to check.

The default maximum is 1000 and can be set via yaml_set_max_nest_level()

• While here, bring in couple more commits in the same code segments to stay up to date: Handle closing flow sequence after explicit key 588eabf (PR 295) & 840b65c (296)

[jpsim]

Thank you!