lib/nbt: Fix a potential memory leak

src/lib/nbt.cpp

@@ -899,10 +899,12 @@ namespace NBT {
 
         ErrorOrVoid List::setElementPointerAt(uint64_t position, Generic* pointer) {
             if (this->tags.size() <= position) {
+                delete pointer;
                 return ErrorOrVoid(true, ErrorCodes::OUT_OF_RANGE);
             }
 
             if (pointer->getTagType() != this->containedType) {
+                delete pointer;
                 return ErrorOrVoid(true, ErrorCodes::INVALID_TYPE);
             }