MDEV-10717 Assertion `!null_value' failed in virtual bool Item::send(Protocol*, String*)

The problem was that null_value was not set to "false" on a well-formed row.
If an ill-formed row was followed by a well-forned row, null_value remained
"true" in the call of Item::send() for the well-formed row.

mysql-test/r/ctype_utf8.result

@@ -5869,5 +5869,31 @@ SELECT length(data) AS len FROM (SELECT REPEAT('ä', 65537) AS data) AS sub;
 len
 131074
 #
+# MDEV-10717 Assertion `!null_value' failed in virtual bool Item::send(Protocol*, String*)
+#
+CREATE TABLE t1 (i INT, KEY(i));
+INSERT INTO t1 VALUES (20081205),(20050327);
+SELECT HEX(i), HEX(CHAR(i USING utf8)) FROM t1;
+HEX(i)	HEX(CHAR(i USING utf8))
+131F197	0131
+1326A35	01326A35
+Warnings:
+Warning	1300	Invalid utf8 character string: 'F197'
+SET sql_mode='STRICT_ALL_TABLES';
+SELECT HEX(i), HEX(CHAR(i USING utf8)) FROM t1;
+HEX(i)	HEX(CHAR(i USING utf8))
+131F197	NULL
+1326A35	01326A35
+Warnings:
+Warning	1300	Invalid utf8 character string: 'F197'
+SELECT CHAR(i USING utf8) FROM t1;
+CHAR(i USING utf8)
+###
+###
+Warnings:
+###	1300	Invalid utf8 character string: 'F197'
+SET sql_mode=DEFAULT;
+DROP TABLE t1;
+#
 # End of 5.5 tests
 #

mysql-test/t/ctype_utf8.test

@@ -1681,6 +1681,20 @@ SELECT length(data) AS len FROM (SELECT REPEAT('ä', 65535) AS data) AS sub;
 SELECT length(data) AS len FROM (SELECT REPEAT('ä', 65536) AS data) AS sub;
 SELECT length(data) AS len FROM (SELECT REPEAT('ä', 65537) AS data) AS sub;
 
+--echo #
+--echo # MDEV-10717 Assertion `!null_value' failed in virtual bool Item::send(Protocol*, String*)
+--echo #
+CREATE TABLE t1 (i INT, KEY(i));
+INSERT INTO t1 VALUES (20081205),(20050327);
+SELECT HEX(i), HEX(CHAR(i USING utf8)) FROM t1;
+SET sql_mode='STRICT_ALL_TABLES';
+SELECT HEX(i), HEX(CHAR(i USING utf8)) FROM t1;
+# Avoid garbage in the output
+--replace_column 1 ###
+SELECT CHAR(i USING utf8) FROM t1;
+SET sql_mode=DEFAULT;
+DROP TABLE t1;
+
 --echo #
 --echo # End of 5.5 tests
 --echo #

sql/item.cc

@@ -5743,6 +5743,7 @@ String *Item::check_well_formed_result(String *str, bool send_error)
   uint wlen= cs->cset->well_formed_len(cs,
                                        str->ptr(), str->ptr() + str->length(),
                                        str->length(), &well_formed_error);
+  null_value= false;
   if (wlen < str->length())
   {
     THD *thd= current_thd;