Uncontrolled memory allocation in MeshEdgebreakerDecoderImpl::DecodeConnectivity allows DoS with tiny input

## Bug

`MeshEdgebreakerDecoderImpl::DecodeConnectivity()` in
`mesh_edgebreaker_decoder_impl.cc:380` reads `num_faces` from untrusted
input and passes it directly to `vector::reserve()` without any
upper-bound validation.

A crafted 61-byte Draco file causes attempted allocation of 3.4+ GB,
crashing any application that decodes untrusted `.drc` files.

## PoC
Base64: RFJBQ08BAAEBXQEBAAB9AAAzMzMzMzMzMzMzMzMzMzMzMzMzMzMzMzMzMzMzMzMzMzMzMzMzMzMzMzMAAA==

## Impact
Denial of service. Any application using Draco to decode untrusted
mesh data (web browsers via WebAssembly, 3D viewers, game engines)
can be crashed with minimal input.

## Suggested Fix
Cap `num_faces` and `num_points` against available buffer size before
calling `reserve()`.

Found via fuzzing with AddressSanitizer.