Resolving a crash when freeing a null pointer when cancelling parses

[logancollins]

This pull request resolves a potential crash we've seen in parser.c.

If a parse is in progress, in certain cases, the lookahead.ptr pointer can be NULL when the code reaches src/parser.c#L. This will only actually crash if the cancellation token happens to have been set to a non-zero value.

This request contains a check to ensure that lookahead.ptr is not NULL before releasing the subtree.

I'm not entirely up on all of the potential branch cases and other code with which this might interact, so I'm happy for a review to ensure this is an acceptable fix!

Thank you!

[maxbrunsfeld]

Thanks for the fix @logancollins; this makes a lot of sense.