Crash: Add dive on empty log

[Dirk Hohndel]

Henrik Brautaset Aronsen <subsurface at henrik.synth.no> writes:

> 1) Open Subsurface (commit 0cf3a11) with 1 dive
> 2) Delete it
> 3) Log » Add Dive
> 4) Crash!
>
> Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
> 0   org.hohndel.subsurface            0x000000010d7b33e8 
> dive_time_widget + 504
> 1   org.hohndel.subsurface            0x000000010d7b3b2b add_new_dive + 43
> 2   org.hohndel.subsurface            0x000000010d7bd465 add_dive_cb + 37
> 3   libgobject-2.0.0.dylib            0x000000010e417104 
> g_closure_invoke + 308
> 4   libgobject-2.0.0.dylib            0x000000010e426859 
> signal_emit_unlocked_R + 1641
> 5   libgobject-2.0.0.dylib            0x000000010e42795c 
> g_signal_emit_valist + 2924
> 6   libgobject-2.0.0.dylib            0x000000010e427f84 g_signal_emit + 116

This one makes sense. When deleting the only dive (which is selected)
and right away adding a dive we don't set amount_selected back to 0 and
end up dereferencing the invalid current_dive.

Nice catch. Will be fixed in my next push.

/D