lib/nbt: Fix a bug in NBT::Helper::writeString()

Writing a string to a byte vector would result in it being replaced
with one holding only the string.

src/lib/nbt.cpp

@@ -364,7 +364,7 @@ namespace NBT {
             if(exportedString.isError){
                 return ErrorOrVoid(true, ErrorCodes::OVERRUN);
             }
-            *destination = exportedString.value;
+            destination->insert(destination->end(), exportedString.value.begin(), exportedString.value.end());
             return ErrorOrVoid();
         }