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>

dive.h

@@ -366,7 +366,7 @@ extern struct dive_table dive_table;
 extern int selected_dive;
 #define current_dive (get_dive(selected_dive))
 
-static inline struct dive *get_dive(unsigned int nr)
+static inline struct dive *get_dive(int nr)
 {
 	if (nr >= dive_table.nr || nr < 0)
 		return NULL;