When ndb_filter_end processes an empty filter (no fields added), it calls
realloc(filter->elem_buf.start, 0) which frees the memory and returns NULL.
The existing code only updated the pointer if realloc
returned non-NULL, leaving elem_buf.start pointing to freed memory. This
caused a double-free crash when ndb_filter_destroy later called free() on
the dangling pointer.
Fix by explicitly setting filter->elem_buf.start to NULL when realloc
returns NULL due to zero-size allocation, and update the assertion to
allow NULL pointers for empty filters. ndb_filter_destroy already checks
for NULL before freeing.
Closes: https://github.com/damus-io/damus/issues/3634
Changelog-Fixed: Fix memory corruption crash when creating empty filters
Signed-off-by: Daniel D’Aquino <daniel@daquino.me>
59498e3256