Bug#848917: nautic FTBFS on architectures where char is unsigned: error: narrowing conversion of '-1' from 'int' to 'char' inside { } [-Wnarrowing]

[Graham Inggs]

Thanks for the upload, Andreas!

If this bug had been tagged [1] like #811740 was, I might have seen it 
sooner.
I only saw it when I noticed nautic was not migrating in Ubuntu. :(

[1] 
https://bugs.debian.org/cgi-bin/pkgreport.cgi?tag=ftbfs-gcc-6;users=debian-...@lists.debian.org