Do not crash in an empty symmetric IncrementalPlot
Do not crash in an empty symmetric IncrementalPlot

This crash could be reproduced by starting QGroundControl, switching to
plot, then clicking the "Symmetric" checkbox.

TheIronBorn/qgroundcontrol,BMP-TECH/qgroundcontrol,jy723/qgroundcontrol,remspoor/qgroundcontrol,scott-eddy/qgroundcontrol,LIKAIMO/qgroundcontrol,LIKAIMO/qgroundcontrol,hejunbok/qgroundcontrol,cfelipesouza/qgroundcontrol,scott-eddy/qgroundcontrol,caoxiongkun/qgroundcontrol,catch-twenty-two/qgroundcontrol,greenoaktree/qgroundcontrol,iidioter/qgroundcontrol,Hunter522/qgroundcontrol,iidioter/qgroundcontrol,josephlewis42/UDenverQGC2,mihadyuk/qgroundcontrol,fizzaly/qgroundcontrol,caoxiongkun/qgroundcontrol,scott-eddy/qgroundcontrol,remspoor/qgroundcontrol,ethz-asl/qgc_asl,nado1688/qgroundcontrol,mihadyuk/qgroundcontrol,josephlewis42/UDenverQGC2,devbharat/qgroundcontrol,catch-twenty-two/qgroundcontrol,greenoaktree/qgroundcontrol,iidioter/qgroundcontrol,nado1688/qgroundcontrol,lis-epfl/qgroundcontrol,greenoaktree/qgroundcontrol,RedoXyde/PX4_qGCS,caoxiongkun/qgroundcontrol,jy723/qgroundcontrol,Hunter522/qgroundcontrol,lis-epfl/qgroundcontrol,catch-twenty-two/qgroundcontrol,kd0aij/qgroundcontrol,cfelipesouza/qgroundcontrol,dagoodma/qgroundcontrol,fizzaly/qgroundcontrol,caoxiongkun/qgroundcontrol,UAVenture/qgroundcontrol,scott-eddy/qgroundcontrol,mihadyuk/qgroundcontrol,catch-twenty-two/qgroundcontrol,LIKAIMO/qgroundcontrol,mihadyuk/qgroundcontrol,remspoor/qgroundcontrol,TheIronBorn/qgroundcontrol,cfelipesouza/qgroundcontrol,RedoXyde/PX4_qGCS,dagoodma/qgroundcontrol,remspoor/qgroundcontrol,CornerOfSkyline/qgroundcontrol,hejunbok/qgroundcontrol,devbharat/qgroundcontrol,dagoodma/qgroundcontrol,ethz-asl/qgc_asl,TheIronBorn/qgroundcontrol,BMP-TECH/qgroundcontrol,hejunbok/qgroundcontrol,iidioter/qgroundcontrol,catch-twenty-two/qgroundcontrol,LIKAIMO/qgroundcontrol,mihadyuk/qgroundcontrol,fizzaly/qgroundcontrol,ethz-asl/qgc_asl,nado1688/qgroundcontrol,dagoodma/qgroundcontrol,scott-eddy/qgroundcontrol,nado1688/qgroundcontrol,nado1688/qgroundcontrol,UAVenture/qgroundcontrol,UAVenture/qgroundcontrol,cfelipesouza/qgroundcontrol,hejunbok/qgroundcontrol,kd0aij/qgroundcontrol,CornerOfSkyline/qgroundcontrol,nado1688/qgroundcontrol,UAVenture/qgroundcontrol,ethz-asl/qgc_asl,greenoaktree/qgroundcontrol,hejunbok/qgroundcontrol,josephlewis42/UDenverQGC2,catch-twenty-two/qgroundcontrol,BMP-TECH/qgroundcontrol,iidioter/qgroundcontrol,CornerOfSkyline/qgroundcontrol,RedoXyde/PX4_qGCS,jy723/qgroundcontrol,remspoor/qgroundcontrol,fizzaly/qgroundcontrol,greenoaktree/qgroundcontrol,dagoodma/qgroundcontrol,BMP-TECH/qgroundcontrol,lis-epfl/qgroundcontrol,jy723/qgroundcontrol,CornerOfSkyline/qgroundcontrol,UAVenture/qgroundcontrol,TheIronBorn/qgroundcontrol,devbharat/qgroundcontrol,cfelipesouza/qgroundcontrol,lis-epfl/qgroundcontrol,greenoaktree/qgroundcontrol,jy723/qgroundcontrol,BMP-TECH/qgroundcontrol,Hunter522/qgroundcontrol,cfelipesouza/qgroundcontrol,scott-eddy/qgroundcontrol,LIKAIMO/qgroundcontrol,RedoXyde/PX4_qGCS,devbharat/qgroundcontrol,RedoXyde/PX4_qGCS,iidioter/qgroundcontrol,kd0aij/qgroundcontrol,josephlewis42/UDenverQGC2,caoxiongkun/qgroundcontrol,lis-epfl/qgroundcontrol,TheIronBorn/qgroundcontrol,Hunter522/qgroundcontrol,devbharat/qgroundcontrol,kd0aij/qgroundcontrol,BMP-TECH/qgroundcontrol,remspoor/qgroundcontrol,mihadyuk/qgroundcontrol,fizzaly/qgroundcontrol,Hunter522/qgroundcontrol,caoxiongkun/qgroundcontrol,ethz-asl/qgc_asl,hejunbok/qgroundcontrol,CornerOfSkyline/qgroundcontrol,devbharat/qgroundcontrol,UAVenture/qgroundcontrol,kd0aij/qgroundcontrol,jy723/qgroundcontrol,kd0aij/qgroundcontrol,fizzaly/qgroundcontrol,RedoXyde/PX4_qGCS,dagoodma/qgroundcontrol,Hunter522/qgroundcontrol,TheIronBorn/qgroundcontrol,CornerOfSkyline/qgroundcontrol,LIKAIMO/qgroundcontrol