patch 9.0.1375: crash when getting member of obj of unknown class

Problem:    Crash when getting member of obj of unknown class.
Solution:   Check for NULL class and give an error message. (Ernie Rael,
            closes #12096)
diff --git a/src/errors.h b/src/errors.h
index f1c36ad..445a9b8 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -3452,4 +3452,6 @@
 #ifdef FEAT_EVAL
 EXTERN char e_cannot_use_non_null_object[]
 	INIT(= N_("E1362: Cannot use a non-null object"));
+EXTERN char e_incomplete_type[]
+	INIT(= N_("E1363: Incomplete type"));
 #endif