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 died silently on a nil pointer.

An additional nil check solves this issue. 
I saw all input spec tests where disabled in the test suite, so I only added the new spec/inputs/gelf.rb to the Makefile.