patch 9.0.0820: memory leak with empty shell command

Problem:    Memory leak with empty shell command.
Solution:   Free the empty string.
diff --git a/src/errors.h b/src/errors.h
index 7aeca75..d3bca1a 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -3330,4 +3330,6 @@
 	INIT(= N_("E1306: Loop nesting too deep"));
 EXTERN char e_argument_nr_trying_to_modify_const_str[]
 	INIT(= N_("E1307: Argument %d: Trying to modify a const %s"));
+EXTERN char e_cannot_resize_window_in_another_tab_page[]
+	INIT(= N_("E1308: Cannot resize a window in another tab page"));
 #endif