Fixes SegFault w/ flashlights & attached power

There was a segfault occuring on
https://github.com/CleverRaven/Cataclysm-DDA/blob/master/src/item.cpp#L12277 when
turning on a heavy-duty flashlight while it is simultaneously connected to a
power source (ie. a vehicle) and does not have a battery.

Added:

-   a short circuit check for ammo to address this issue in the future in
    case there are any other edge cases where this occurs.
-   a check for a linked power source so the light source uses the
    power source's linked power when determining the illumination value.

Fixes #78580

Note: I used https://github.com/CleverRaven/Cataclysm-DDA/blob/master/src/item.cpp#L14177
as a reference for the linked power check above. This means the two power checks are extremely similar
and it makes me believe that it would be beneficial to have a function such as
`has_link_power` that returns a boolean (true for has power, false otherwise). My only
hesitation is that I do not know where else that sort of power check is done and what the complete
scope of the checks against `charge_rate` / `charge_interval` are for.

src/item.cpp

@@ -12273,6 +12273,18 @@ int item::getlight_emit() const
         return 0;
     }
     if( has_flag( flag_CHARGEDIM ) && is_tool() && !has_flag( flag_USE_UPS ) ) {
+        // check if there's a link and it has a charge rate or charge interval
+        // if so, use its power supply
+        if( has_link_data() && ( link().charge_rate > 0 || link().charge_interval > 0 ) ) {
+            return lumint;
+        }
+
+        // If we somehow got this far without a battery/power,
+        // return an illumination value of 0
+        if( !has_ammo() ) {
+            return 0;
+        }
+
         // Falloff starts at 1/5 total charge and scales linearly from there to 0.
         const ammotype &loaded_ammo = ammo_data()->ammo->type;
         if( ammo_capacity( loaded_ammo ) &&