Changes

gsmtap_util: Avoid sink_fd leak gsmtap_source_add_sinki() called multiple times

gti->sink_fd could leak if  gsmtap_source_add_sink() was called multiple
times.

Also if gsmtap_source_add_sink_fd() failed, rc != -1 was returned and
hence a close() on a wrong fd would be attempted.

This commit fixes both issues, making the whole sink_fd field much more
robust.

Change-Id: I7af5a6c7d64954ee2cc013711702b846dfaa02b1