Introduce _cleanup_(strbuf_cleanupp) and use it to fix null deref on error

catalog_update() would call strbuf_cleanup(NULL) on allocation error.
CID #1390928.

src/basic/strbuf.h

@@ -11,6 +11,8 @@
 #include <stdint.h>
 #include <sys/types.h>
 
+#include "macro.h"
+
 struct strbuf {
         char *buf;
         size_t len;
@@ -40,3 +42,4 @@ struct strbuf *strbuf_new(void);
 ssize_t strbuf_add_string(struct strbuf *str, const char *s, size_t len);
 void strbuf_complete(struct strbuf *str);
 void strbuf_cleanup(struct strbuf *str);
+DEFINE_TRIVIAL_CLEANUP_FUNC(struct strbuf*, strbuf_cleanup);