Prevent out-of-range memory writes by sparse accessors

This turned up during fuzz testing. Corrupted data would make assimp write to random memory locations, leading to subsequent crashes.

code/AssetLib/glTF2/glTF2Asset.inl

@@ -809,6 +809,11 @@ inline void Accessor::Sparse::PatchData(unsigned int elementSize) {
         }
 
         offset *= elementSize;
+
+        if (offset + elementSize > data.size()) {
+            throw DeadlyImportError("Invalid sparse accessor. Byte offset for patching points outside allocated memory.");
+        }
+
         std::memcpy(data.data() + offset, pValues, elementSize);
 
         pValues += elementSize;