Bug #38701: Crash in String::append when inserting duplicate empty strings an uft8

SET col
                  
When reporting a duplicate key error the server was making incorrect assumptions 
on what the state of the value string to include in the error is.

Fixed by accessing the data in this string in a "safe" way (without relying on it
having a terminating 0).
      
Detected by code analysis and fixed a similar problem in reporting the foreign key
duplicate errors.

mysql-test/r/type_set.result

@@ -93,4 +93,14 @@ c
 1,2,3
 64
 DROP TABLE t1;
+CREATE TABLE t1 (
+set_unique_utf8 set ('a','b','c','d','e','f','g','h','i','j','k','l',
+'m','n','o','p','q','r','s','t','u','v','w','x',
+'y','z') CHARACTER SET utf8,
+unique (set_unique_utf8)
+);
+INSERT INTO t1 ( set_unique_utf8 ) VALUES ( '' );
+INSERT INTO t1 ( set_unique_utf8 ) VALUES ( '' );
+ERROR 23000: Duplicate entry '' for key 'set_unique_utf8'
+DROP TABLE t1;
 End of 5.0 tests

mysql-test/t/type_set.test

@@ -75,4 +75,23 @@ INSERT INTO t1 VALUES(9223372036854775808);
 SELECT * FROM t1;
 DROP TABLE t1;
 
+#
+# Bug #38701: Crash in String::append when inserting duplicate empty strings
+# an uft8 SET col
+#
+
+CREATE TABLE t1 (
+        set_unique_utf8 set ('a','b','c','d','e','f','g','h','i','j','k','l',
+                             'm','n','o','p','q','r','s','t','u','v','w','x',
+                             'y','z') CHARACTER SET utf8,
+        unique (set_unique_utf8)
+);
+
+INSERT INTO t1 ( set_unique_utf8 ) VALUES ( '' );
+--error ER_DUP_ENTRY 
+INSERT INTO t1 ( set_unique_utf8 ) VALUES ( '' );
+
+DROP TABLE t1;
+
+
 --echo End of 5.0 tests

sql/handler.cc

@@ -2496,7 +2496,7 @@ void handler::print_keydup_error(uint key_nr, const char *msg)
       str.append(STRING_WITH_LEN("..."));
     }
     my_printf_error(ER_DUP_ENTRY, msg,
-		    MYF(0), str.c_ptr(), table->key_info[key_nr].name);
+		    MYF(0), str.c_ptr_safe(), table->key_info[key_nr].name);
   }
 }
 
@@ -2564,7 +2564,7 @@ void handler::print_error(int error, myf errflag)
         str.append(STRING_WITH_LEN("..."));
       }
       my_error(ER_FOREIGN_DUPLICATE_KEY, MYF(0), table_share->table_name.str,
-        str.c_ptr(), key_nr+1);
+        str.c_ptr_safe(), key_nr+1);
       DBUG_VOID_RETURN;
     }
     textno= ER_DUP_KEY;