Decrement refcount before calling free in to_next_sibling

Fixes an assertion in debug builds which I accidentally
introduced when attending the review comments for [1]
in [2] - instead of only removing the increment, I also
removed the decrement which was wrong - `std::mem::forget`
only allows us to remove the increment, but the decrement
is still needed before free since we are in a place of
code that is by definition only run when the rc value is set to 1.
See also `can_take_ptr`.

I did not spot this earlier since I ran the integration test
on a release build, where the assertion was disabled. It's sad
that the rowan repo itself doesn't have any big test coverage
in this repo itself, but rather relies on external repos for
testing purposes...

[1]: #171 (comment)
[2]: https://github.com/rust-analyzer/rowan/compare/60a632ad984ab451e32058169193511154c675a9..ab5463e2749330be6846886c21e98c83caca8598

Fixes: #172

src/cursor.rs

@@ -790,6 +790,7 @@ impl SyntaxNode {
                 Some(SyntaxNode { ptr })
             })
             .or_else(|| {
+                data.dec_rc();
                 unsafe { free(ptr) };
                 None
             })
@@ -1234,6 +1235,7 @@ impl SyntaxElement {
                 }
             })
             .or_else(|| {
+                data.dec_rc();
                 unsafe { free(ptr) };
                 None
             })