- check malloc/calloc/realloc results in constructors, the JSON and
  OpenStep writers, the XML text-part helpers and base64 decoding, and
  return PLIST_ERR_NO_MEM instead of asserting or dereferencing NULL
- guard size calculations against overflow in the UTF-8/UTF-16BE
  conversions, ptrarray, and bytearray
- make hash_table_insert() report failure instead of silently dropping
  the entry; writers propagate PLIST_ERR_NO_MEM, and dict lookup caches
  are discarded so lookups fall back to a linear search
- don't overwrite an earlier parse error code in the XML parser, and
  report PLIST_ERR_NO_MEM when reading text content fails for lack of memory
- OpenStep parser: check node/data/string allocations and dict/array
  insertion results

Based on #323 by bg0d-droid.

Co-authored-by: bg0d-droid <81825610+bg0d-droid@users.noreply.github.com>

ptr_array_insert() assigned the result of realloc() directly to
pa->pdata and bumped the capacity without checking for failure, so on
out-of-memory the old buffer was leaked and the following store or
memmove() wrote through a NULL pointer.

Keep the old buffer on failure and return -1 from ptr_array_insert()
and ptr_array_add(). The callers now handle the error:

- the array lookup cache (plist.c) is dropped instead of silently going
  out of sync with the node list; lookups then fall back to walking
  the children, as they do before the cache exists
- bplist parsing (used_indexes) and serialization (objects) return
  PLIST_ERR_NO_MEM; ignoring the error in parse_bin_node_at_index()
  would otherwise loop forever since the array never grows

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>