Fix crash when applying a non-edit of nicknames

As reported by Henrik, there was a crash lurking in edit_dc_nickname code.
Invoking the dialog and selecting apply without changes exposed it neatly.

This is a one line fix.

Signed off by Amit Chaudhuri <amit.k.chaudhuri@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
divelist.c:icon_click_cb(): check if a GtkTreePath is found

In icon_click_cb() we need to check if a correct GtkTreePath is found
(using gtk_tree_view_get_path_at_pos()) before requesting a GtkTreeIter
for it.

Without this patch a bug is reproducible, where the user may click
outside of the GtkTreeView entries, but still in the GtkTreeView -
e.g. when only one entry is available.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>