CVE-2026-54610: LibreDWG: const-input mutation and crash in bit_TV_to_utf8 error handling

LibreDWG is a free C library to read and write DWG files. Prior to 0.14, bitTVtoutf8() in src/bits.c can write to the caller-provided src buffer despite accepting const input. On an iconv() error other than E2BIG, the function frees its output buffer and passes the original input alias to bituexpand(), which expands or shrinks Unicode and multibyte escape sequences in place. If the input is a string literal, read-only mapping, or other immutable buffer, the write can cause a segmentation fault and denial of service; mutable caller-owned input can be unexpectedly modified before being returned. This affects applications that expose the conversion API to untrusted input under those conditions. Direct file-triggered exploitability through normal DWG decode/export paths remains unconfirmed because many TV strings are heap allocated by bitreadTV(). This issue is fixed in version 0.14.