Check if multi-dive editing is actually needed

Check if multi-dive editing is actually needed

Context menu callbacks always invoke edit_multi_dive_info(-1) instead of
edit_dive_info.  Since -1 means "all selected", it was impossible to edit
dive notes through the context menus.  This commit makes the function
check if multiple dives are actually selected.

Signed-off-by: Mikko Rasa <tdb@tdb.fi>