rchk issue on CRAN

[Rich FitzJohn]

https://raw.githubusercontent.com/kalibera/cran-checks/master/rchk/results/thor.out

Package thor version 1.0.1
Package built using 76078/R 3.6.0; x86_64-pc-linux-gnu; 2019-02-10 10:19:24 UTC; unix   
Checked with rchk version c132302c5d39a55c436a06df73e539a1bace7486
More information at https://github.com/kalibera/cran-checks/blob/master/rchk/PROTECT.md

Function mdb_reader_list_callback
  [PB] has possible protection stack imbalance thor/src/thor.c:461