update block.get_field, block.set_field and fields-related docs

src/data/StructLayout.cpp

@@ -372,6 +372,12 @@ void StructLayout::deserialize(const dv::value& src) {
         
         int elements = 1;
         fieldmap.at("length").get(elements);
+        if (elements <= 0) {
+            throw std::runtime_error(
+                "invalid field " + util::quote(name) + " length: " +
+                std::to_string(elements)
+            );
+        }
 
         auto convertStrategy = FieldConvertStrategy::RESET;
         if (fieldmap.has("convert-strategy")) {

src/logic/scripting/lua/libblock.cpp

@@ -489,6 +489,10 @@ static int l_get_field(lua::State* L) {
     if (field == nullptr) {
         return 0;
     }
+    if (index >= field->elements) {
+        throw std::out_of_range(
+            "index out of bounds [0, "+std::to_string(field->elements)+"]");
+    }
     const ubyte* src = chunk->blocksMetadata.find(voxelIndex);
     if (src == nullptr) {
         return 0;
@@ -553,6 +557,10 @@ static int l_set_field(lua::State* L) {
     if (field == nullptr) {
         return 0;
     }
+    if (index >= field->elements) {
+        throw std::out_of_range(
+            "index out of bounds [0, "+std::to_string(field->elements)+"]");
+    }
     ubyte* dst = chunk->blocksMetadata.find(voxelIndex);
     if (dst == nullptr) {
         dst = chunk->blocksMetadata.allocate(voxelIndex, dataStruct.size());