MDEV-6788: The variable 'role' is being used without being initialized at sql_acl.cc:8840

Don't check the value of 'role' variable in the cases where we don't need it. (it 
may be marked as unitialized and we get a runtime error).

sql/sql_acl.cc

@@ -8645,7 +8645,7 @@ static int handle_grant_struct(enum enum_acl_lists struct_no, bool drop,
   int elements;
   const char *UNINIT_VAR(user);
   const char *UNINIT_VAR(host);
-  const char *UNINIT_VAR(role);
+  const char *role;
   ACL_USER *acl_user= NULL;
   ACL_ROLE *acl_role= NULL;
   ACL_DB *acl_db= NULL;
@@ -8795,8 +8795,6 @@ static int handle_grant_struct(enum enum_acl_lists struct_no, bool drop,
       user= "";
     if (! host)
       host= "";
-    if (! role)
-      role= "";
 
 #ifdef EXTRA_DEBUG
     DBUG_PRINT("loop",("scan struct: %u  index: %u  user: '%s'  host: '%s'",
@@ -8805,6 +8803,8 @@ static int handle_grant_struct(enum enum_acl_lists struct_no, bool drop,
 
     if (struct_no == ROLES_MAPPINGS_HASH)
     {
+      if (! role)
+        role= "";
       if (user_from->is_role() ? strcmp(user_from->user.str, role) :
           (strcmp(user_from->user.str, user) ||
            my_strcasecmp(system_charset_info, user_from->host.str, host)))