Historically, `free(3)`-style functions have been NULL-safe, to make
resource handling functions more robust. This doesn't seem to be the
case with tree-sitter's `ts_tree_delete` and `ts_parser_delete` C
functions, which immediately dereference the passed pointer. This
leads to complexity in client libraries that have to ensure that trees
and parsers are cleaned up correctly.
This patch adds NULL checks to `parse_delete` and `tree_delete`. They
should have negligable performance impacts, since null checks are fast.
I didn't change the internal _delete functions, as arguably those
being NULL-unsafe is a feature, not a bug.
