TPSPencilBeam should not segfault if user config results in zero spots

[djboersma]

When the TPS plan text file contains just one "field", and the user disables that field with the "setNotAllowedFieldID" option of the TPSPencilBeam actor, then the TPSPencilBeam actor runs into a segfault. It should instead print an informative error message and Gate should then exit with a nonzero return code. Thanks to @mbferruz for reporting.