lib/igc: Don't break on missing logger information
lib/igc: Don't break on missing logger information

Some old Volkslogger firmware produced valid files that are missing
the first A line

skylines-project/skylines,Harry-R/skylines,RBE-Avionik/skylines,Harry-R/skylines,snip/skylines,skylines-project/skylines,skylines-project/skylines,kerel-fs/skylines,Harry-R/skylines,skylines-project/skylines,shadowoneau/skylines,Turbo87/skylines,TobiasLohner/SkyLines,dkm/skylines,shadowoneau/skylines,Turbo87/skylines,kerel-fs/skylines,TobiasLohner/SkyLines,shadowoneau/skylines,snip/skylines,kerel-fs/skylines,RBE-Avionik/skylines,dkm/skylines,dkm/skylines,snip/skylines,TobiasLohner/SkyLines,Turbo87/skylines,RBE-Avionik/skylines,shadowoneau/skylines,dkm/skylines,Turbo87/skylines,Harry-R/skylines,RBE-Avionik/skylines