The gr-diff-cursor#getAddress method returns null when it has no
position or no address corresponds to it's position. However, the
gr-diff-view#_onLineSelected method did not account for this case and
attempted to use null addresses to construct URLs when moving the cursor
to a "File" line of a diff.
With this change, the diff view neither uses line number nor the diff
side when constructing URLs if the cursor does not yield an address.
Change-Id: I628658295bca1f49e0c2d3484e2e0d01e71bcd91
1bd466a24b