hls: only set DISCONT flag on a valid buffer

hls: only set DISCONT flag on a valid buffer

Recent refactoring causes this code to be called with either a NULL
fragment, or a non NULL fragment. In the former case, we don't have
a buffer. In the latter case, the original code dealing with DISCONT
assumed the buffer was valid. Testing for a NULL buffer here thus
does not seem to change the intent, and fixes:

Coverity 1195147