Give robot a volume

This is a bit odd, but as far as I can tell volume was never initialized
(UB), but had a non-zero value and so did not trigger any issues with
the test.

Now that optional always initializes it (to 0), it produced issues. Just
assign it to 100 L.

Author: ehughsbaird

data/json/monsters/defense_bot.json

@@ -289,6 +289,7 @@
     "diff": 100,
     "hp": 50,
     "speed": 140,
+    "volume": "100 L",
     "material": [ "steel" ],
     "symbol": "@",
     "color": "red",

src/item.cpp

@@ -7480,7 +7480,7 @@ units::volume item::corpse_volume( const mtype *corpse ) const
     if( corpse_volume > 0_ml ) {
         return corpse_volume;
     }
-    debugmsg( "invalid monster volume for corpse" );
+    debugmsg( "invalid monster volume for corpse of %s", corpse->id.str() );
     return 0_ml;
 }
 

tests/item_test.cpp

@@ -332,6 +332,7 @@ TEST_CASE( "item_length_sanity_check", "[item]" )
 TEST_CASE( "corpse_length_sanity_check", "[item]" )
 {
     for( const mtype &type : MonsterGenerator::generator().get_all_mtypes() ) {
+        INFO( type.id.str() )
         const item sample = item::make_corpse( type.id );
         assert_minimum_length_to_volume_ratio( sample );
     }