Bug#26571995 SET DEFAULT ROLE ALL don't set mandatory roles.

Post-push fix for UBSAN warning:
runtime error:
null pointer passed as argument 1, which is declared to never be null
sql/auth/sql_authorization.cc:7525

To repeat: ./mtr --mem --sanitize json_table

(cherry picked from commit 01fee7c1eb7aa2d516dab2e2d3e678056ad6ee5e)

sql/auth/sql_authorization.cc

@@ -7522,5 +7522,8 @@ bool operator==(std::pair<const Role_id, std::pair<std::string, bool> > &a,
 
 bool operator==(const LEX_CSTRING &a, const LEX_CSTRING &b)
 {
-  return (a.length == b.length && memcmp(a.str, b.str, a.length) == 0);
+  return
+    (a.length == b.length &&
+     ( (a.length == 0) ||
+       (memcmp(a.str, b.str, a.length) == 0)) );
 }