LibreDWG: excessive class-decoding reallocation in R13-R2000 DWG parsing

LibreDWG is a free C library to read and write DWG files. Prior to 0.14, a specially crafted R13-R2000 DWG can trigger algorithmic resource exhaustion in `decode_R13_R2000` in `src/decode.c`. The class-decoding loop performs `realloc` on every iteration without a sanity check on `num_classes`. Manipulating that field forces an extremely slow allocation loop and denial of service when the file is parsed. Running the parser in a sandbox with memory limits is a workaround rather than a remediation. This issue is fixed in version 0.14.