Leica: prevent IOOB 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.
Leica: more sensible filter/channel linking

Only attempts to link to channels that exist, rather than attempting to link
all channels and catching an exception if that fails.
Merge pull request #1154 from melissalinkert/leica-filter

Leica: prevent exception when linking EmissionFilter