Leica: prevent exception when linking EmissionFilter

This prevents the reader from attempting to link an EmissionFilter to e.g. Channel:1 before Channel:0.

See QA 9255.  To test, verify that an IndexOutOfBoundsException occurs when the .lei file from the larger dataset is initialized (by opening in ImageJ, or ```showinf -nopix -omexml```) without this PR's fix.  With this PR, the file should open normally and ```showinf -nopix -omexml``` should display metadata and OME-XML,