makePie fails with only 1 signature weight non-zero and/or with unknown=0

[tedtoal]

If makePie() is called when only one signature weight is non-zero, it fails due to a needed ", drop=FALSE". If furthermore it is called after that fix is added, with only one signature and with unknown=0, it also fails in the palette() function call (which can be fixed by adding a dummy second color to colors.sigs.present).