libde265 has an out-of-bounds write in process_reference_picture_set via predicted short-term RPS

libde265 is an open source implementation of the h.265 video codec. Prior to version 1.0.20, a crafted H.265 bitstream can cause an out-of-bounds array write in `decoder_context::process_reference_picture_set()` (`libde265/decctx.cc:1376`). The root cause is a missing aggregate bound check on predicted short-term reference picture set entries. Individual list sizes are validated, but the combined count after predicted RPS construction can exceed the 16-entry `PocStFoll` array, writing at index 16. Version 1.0.20 patches the issue.