Bug#925749: Problem not reproducible using g++9.2.1

[Feng Zhao]

I test the packaging on 1.3.1+dfsg-2 with g++9.2.1. The compilation is 
ok but one of the test fails:

liblemon/obj-x86_64-linux-gnu/test$ ./lgf_reader_writer_test
terminate called after throwing an instance of 'lemon::FormatError'
  what():  lemon:FormatError: Item not found