Fix crash in df.global:_field() when global address is unknown

and add a test

library/LuaTypes.cpp

@@ -664,6 +664,9 @@ static int meta_global_field_reference(lua_State *state)
     auto field = (struct_field_info*)find_field(state, 2, "reference");
     if (!field)
         field_error(state, 2, "builtin property or method", "reference");
+    void *ptr = *(void**)field->offset;
+    if (!ptr)
+        field_error(state, 2, "global address not known", "reference");
     field_reference(state, field, *(void**)field->offset);
     return 1;
 }

test/structures/globals.lua

@@ -0,0 +1,33 @@
+config.mode = 'title'
+config.target = 'core'
+
+local function with_temp_global_address(name, addr, callback, ...)
+    dfhack.call_with_finalizer(2, true,
+        dfhack.internal.setAddress, name, dfhack.internal.getAddress(name),
+        function(...)
+            dfhack.internal.setAddress(name, addr)
+            callback(...)
+        end, ...)
+end
+
+function test.unknown_global_address()
+    expect.ne(dfhack.internal.getAddress('army_next_id'), 0)
+    local old_id = df.global.army_next_id
+
+    with_temp_global_address('army_next_id', 0, function()
+        expect.error_match('Cannot read field global.army_next_id: global address not known.', function()
+            local _ = df.global.army_next_id
+        end)
+
+        expect.error_match('Cannot write field global.army_next_id: global address not known.', function()
+            df.global.army_next_id = old_id
+        end)
+
+        expect.error_match('Cannot reference field global.army_next_id: global address not known.', function()
+            local _ = df.global:_field('army_next_id')
+        end)
+    end)
+
+    expect.gt(dfhack.internal.getAddress('army_next_id'), 0)
+    expect.eq(df.global.army_next_id, old_id)
+end