plist_from_openstep() tested ctx.err > 0 after parsing the plist as
'strings' data, but error codes are negative, so a failure was never
detected and the function returned success with a NULL plist (leaking the
temporary dict). Check for any error and return it.

- 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>

byte_array_grow() assigned the result of realloc() directly to
ba->data and increased the capacity even if realloc() failed, so the
following memcpy() in byte_array_append() wrote to NULL + len.

On failure, free the old buffer and leave the byte array in a failed
state (data == NULL), which byte_array_append() already ignores. The
callers check for that state:

- the writers (bin, xml, json, openstep and the text output formats)
  return PLIST_ERR_NO_MEM instead of handing out a NULL/truncated buffer
- node_to_xml() base64-encodes <data> directly into the grown buffer,
  so it must bail out there (only guarded by assert() before)
- the OpenStep parser returns PLIST_ERR_NO_MEM for <hex data> instead of
  silently returning truncated data

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

Avoid undefined behavior when serializing malformed PLIST_DATE values
containing NaN, infinity, or values outside the Time64_T range. Add a
shared helper for checked date conversion and use it across writer paths.

- Use PLIST_OPT_COERCE option to coerce PLIST_BOOLEAN, PLIST_DATE, PLIST_UID, and PLIST_NULL to OpenStep-compatible types (1 or 0, ISO 8601 strings, integers, and 'NULL' string)
- Add plist_to_openstep_with_options() function to allow passing coercion option (and others)
- Update plist_write_to_string() and plist_write_to_stream() accordingly

Thanks to @unbengable12 for reporting. Addresses #288, #289, #290, #291, and #292.

Thanks to @LkkkLxy for pointing out the issue.

Thanks to @ylwango613 for pointing this out!