Fix: GPv3: Crash converting vertex groups

A grease pencil test file
(can be found here: https://cloud.blender.org/p/gallery/5d9c6ee20f9018baaca071c3)
was crashing when converting to GPv3.
The index of a deform weight group was out of bounds.
We will have to investgate how this can happen, but
in the conversion code it seems best to ignore and skip over
invalid indices.

Pull Request: https://projects.blender.org/blender/blender/pulls/120170

source/blender/blenkernel/intern/grease_pencil_convert_legacy.cc

@@ -172,6 +172,10 @@ static void find_used_vertex_groups(const bGPDframe &gpf,
     Span<MDeformVert> dverts = {gps->dvert, gps->totpoints};
     for (const MDeformVert &dvert : dverts) {
       for (const MDeformWeight &weight : Span<MDeformWeight>{dvert.dw, dvert.totweight}) {
+        if (weight.def_nr >= dvert.totweight) {
+          /* Ignore invalid deform weight group indices. */
+          continue;
+        }
         is_group_used[weight.def_nr] = true;
       }
     }
@@ -362,6 +366,10 @@ void legacy_gpencil_frame_to_grease_pencil_drawing(const bGPDframe &gpf,
     dst_dvert.dw = static_cast<MDeformWeight *>(MEM_dupallocN(src_dvert.dw));
     const MutableSpan<MDeformWeight> vertex_weights = {dst_dvert.dw, dst_dvert.totweight};
     for (MDeformWeight &weight : vertex_weights) {
+      if (weight.def_nr >= dst_dvert.totweight) {
+        /* Ignore invalid deform weight group indices. */
+        continue;
+      }
       /* Map def_nr to the reduced vertex group list. */
       weight.def_nr = stroke_def_nr_map[weight.def_nr];
     }