Start cleaning up dive accessors

I'm going to add a menu to import (and eventually export) dives, and so
we'd like to be able to start out with no dives at all.  Right now we
croak if that happens - it's not like the code has been written with
actual end users in mind.

So start cleaning things up.  First make the 'current_dive' macro work
right even for invalid dives.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

display.h

@@ -6,12 +6,12 @@
 #include <cairo.h>
 
 extern int selected_dive;
+#define current_dive (get_dive(selected_dive))
+
 extern GtkWidget *dive_profile_frame(void);
 extern GtkWidget *dive_info_frame(void);
 extern GtkWidget *create_dive_list(void);
 extern void update_dive_info(struct dive *dive);
 extern void repaint_dive(void);
 
-#define current_dive (dive_table.dives[selected_dive])
-
 #endif