libde265 has an unbounded memory leak via orphaned slice headers in `read_slice_NAL`

libde265 is an open source implementation of the h.265 video codec. Prior to version 1.0.20, a crafted sequence of H.265 NAL units causes `decoder_context::read_slice_NAL()` (`libde265/decctx.cc:481`) to attach slice headers to a finished picture object that has no active image unit, resulting in attacker-controlled unbounded heap growth. The retained headers are never freed until the picture is released, which may not happen during continuous streaming. Version 1.0.20 patches the issue.