Fix GC buffer unnecessarily growing up to GC_MAX_BUF_SIZE when scanning WeakMaps - #13572

This should fix #13569.

This is caused by a wrong condition in gc_extra_root() causing it to grow the root buffer when it's not necessary: It grows the buffer when the first_unused slot is higher or equal to gc_threshold. Additionally, this condition applies repeatedly because the threshold is (legitimately) not updated in this case, thus the buffer size quickly grows towards GC_MAX_BUF_SIZE, after which an E_WARNING is triggered and the GC is disabled.

The fix is to compare first_unused to buf_size instead.