Fix some of the problems reported by cppcheck

Thanks to Christian for running the static code analysis tool against
subsurface...

There were some false positives, a few style issues that I'll ignore for
now, and two actual potential bugs.

First: Don't check unsigned variables for < 0

This has been around for a while and we are lucky that while technically a
bug it still works as expected. Passing a negative idx simply turns it
into a very large unsigned integer which then fails the > dive_table.nr
test. So it still gets a NULL returned. A bug? Yes. Critical? No.

Mismatched allocation and free

This is an actual bug that potentially could cause issues. We allocate
memory with malloc and free it with g_free. Not good.

Reported-by: Cristian Ionescu-Idbohrn <cristian.ionescu-idbohrn@axis.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

divelist.c

@@ -110,7 +110,7 @@ void dump_selection(void)
 	int i;
 	struct dive *dive;
 
-	printf("currently selected are %d dives:", amount_selected);
+	printf("currently selected are %u dives:", amount_selected);
 	for_each_dive(i, dive) {
 		if (dive->selected)
 			printf(" %d", i);