Fix gelf input for handling of chunked messages

Between 1.1.13 and 1.2.1 , the gelf input lost it's ability to handle chunked messages (multiple UDP packets) .
The gelf input dies on a nil pointer when a chunk arrives.

An additional nil check to skip the non-final chunks solves this issue. 
I saw that all input spec tests where disabled in the test suite, so I only added the new spec/inputs/gelf.rb to the Makefile.