dxf-g: Fix heap-buffer-overflow in polyline face generation

[Meghna45-code]

This Pull Request fixes the heap-buffer-overflow crash described in BRL-CAD #228 .

The Issue:
1.When parsing a DXF file, BRL-CAD reads physical coordinates (corners) and stores them sequentially in a heap-allocated list called polyline_vert_indices.
2.When parsing a face connection instruction (using a vertex flagged with POLY_VERTEX_FACE), the translator reads the corner indices from the DXF file and saves them to the face array.
3.To build the mesh, BRL-CAD connects these corners by looking up the actual coordinates in the list: polyline_vert_indices[face[i] - 1]

The Vulnerability: The translator trusted the DXF file completely and did not check if the index specified (face[i]) was valid. If the DXF file contains an invalid ID (such as 0 which leads to index -1, or a very large index like 99999 when only a few vertices are defined), the program attempted to access memory outside the bounds of the polyline_vert_indices array, causing a heap-buffer-overflow and immediate crash.

I modified the process_entities_polyline_vertex_code function in dxf-g.c to validate the input indices before passing them to
add_triangle:

1.Added bounds checks to ensure every parsed corner index (face[0], face[1], face[2], face[3]) is >= 1 and <= polyline_vert_indices_count.
2. If a face contains an out-of-bounds index, BRL-CAD now logs a warning and safely skips drawing that triangle instead of accessing invalid memory and crashing.