Fix a few cases of Clang 10 with UBSAN detecting undefined behavior

Clang 10 considers adding any offset, including 0, to the null pointer
to be undefined behavior. `(void *)NULL + 0 = kaboom`.

lib/src/query.c

@@ -1252,6 +1252,9 @@ const TSQueryPredicateStep *ts_query_predicates_for_pattern(
 ) {
   Slice slice = self->predicates_by_pattern.contents[pattern_index];
   *step_count = slice.length;
+  if (self->predicate_steps.contents == NULL) {
+    return NULL;
+  }
   return &self->predicate_steps.contents[slice.offset];
 }