Prevent crash on trying to inline an empty list

When doing stupid things, such as blacklisting all items in the game,
the game would crash on loading trying to inline an empty list of
requirements. Check that that list is not empty before attempting to
do anything with it.

src/requirements.cpp

@@ -485,7 +485,9 @@ void inline_requirements( std::vector< std::vector<T> > &list, Getter getter )
             iter = vec.erase( iter );
 
             const auto &to_inline = getter( multiplied );
-            vec.insert( iter, to_inline.front().begin(), to_inline.front().end() );
+            if( !to_inline.empty() ) {
+                vec.insert( iter, to_inline.front().begin(), to_inline.front().end() );
+            }
         }
     }
 }