sp_lvl: fix memory leak on lspo_region

If tutorial is entered, we get following leak on exit:

=================================================================
==81358==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 96 byte(s) in 3 object(s) allocated from:
    #0 0x7f6996edefdf in __interceptor_malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:69
        #1 0x5601c255bcbb in alloc /home/miku/src/NetHack/src/alloc.c:71

Indirect leak of 5064 byte(s) in 3 object(s) allocated from:
    #0 0x7f6996edefdf in __interceptor_malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:69
        #1 0x5601c255be1e in alloc /home/miku/src/NetHack/src/alloc.c:71
	    #2 0x5601c255be1e in dupstr /home/miku/src/NetHack/src/alloc.c:236

SUMMARY: AddressSanitizer: 5160 byte(s) leaked in 6 allocation(s).

Fix this by freeing the cloned selection before returning.

src/sp_lev.c

@@ -6191,6 +6191,8 @@ lspo_region(lua_State *L)
             selection_do_grow(sel, W_ANY);
         selection_iterate(sel, sel_set_lit, (genericptr_t) &rlit);
 
+        selection_free(sel, TRUE);
+
         /* TODO: skip the rest of this function? */
         return 0;
     } else {