add guards for h->otl == NULL before calling otlSubtableAdd

[josh-hadley]

Description

Add guards for h->otl == NULL in GSUB and GPOS code. Previously this was not explicitly checked, but could be NULL resulting in segfault.

Checklist:

• [x] I have followed the Contribution Guidelines
• [ ] I have added test code and data to prove that my code functions correctly
• [x] I have verified that new and existing tests pass locally with my changes
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation