patch 7.4.1096

Problem:    Need several lines to verify a command produces an error.
Solution:   Add assert_fails(). (suggested by Nikolay Pavlov)
            Make the quickfix alloc test actually work.

src/alloc.h

@@ -17,4 +17,5 @@ typedef enum {
 	aid_qf_namebuf,
 	aid_qf_errmsg,
 	aid_qf_pattern,
+	aid_last,
 } alloc_id_T;