Avoid crash when clockwork bee falls in deep water

Prevents a segfault which would happen when the bee's target disappears
from view at the same time as the bee is flying over a body of deep
water (or lava). The case that the "inert" (and flightless) bee failed
to place, due to no solid ground being near enough, wasn't handled and
execution fell through to the next block where it was assumed a target
did exist.

Rather than needing to figure out exactly why the bee won't place to
have messages like splashing in the water, the bee can just self-
destruct; it's the same result.

crawl-ref/source/spl-summoning.cc

@@ -3182,10 +3182,10 @@ void handle_clockwork_bee_spell(int turn)
     if (!targ || !targ->alive() || !targ->visible_to(&you)
         || !you.see_cell(targ->pos()))
     {
-        targ = _get_clockwork_bee_target();
+        auto new_targ = _get_clockwork_bee_target();
 
         // Couldn't find anything, so just deposit an inert bee near us
-        if (!targ)
+        if (!new_targ)
         {
             mgen_data mg = _pal_data(MONS_CLOCKWORK_BEE_INACTIVE,
                                         random_range(80, 120),
@@ -3200,10 +3200,20 @@ void handle_clockwork_bee_spell(int turn)
                 // reactivate us.
                 if (targ)
                     bee->props[CLOCKWORK_BEE_TARGET].get_int() = targ->mid;
-
-                return;
             }
+            else
+            {
+                // Unable to create inert bee, probably the player was flying
+                // over deep water / lava. Rather than try to work out what
+                // happened, just self destruct.
+                mprf("Without a target and with nowhere to land, your clockwork "
+                     "bee falls apart in a shower of cogs and coils.");
+                check_place_cloud(CLOUD_DUST, mg.pos, random_range(1,2), &you, 0, -1);
+            }
+            return;
         }
+
+        targ = new_targ;
     }
 
     mgen_data mg = _pal_data(MONS_CLOCKWORK_BEE, random_range(400, 500),