Bug #28815 apparent memory corruption in "show warnings" output

Description:
When running a select with some unions, and a subselect, there are warnings reported.

The look strangely corrupted:

<big select statement>

Empty set, 613 warnings (0.02 sec)

mysql> show warnings;
+---------+------+----------------------------------------------------------------------
| Level   | Code | Message
+---------+------+----------------------------------------------------------------------
| Warning | 1292 | Incorrect datetime value: '?' for column 'col101' at row 1
| Warning | 1292 | Incorrect date value: '?' for column 'col27' at row 1
| Warning | 1292 | Incorrect datetime value: '?' for column 'col15' at row 1
<d$`¶▲???-d?↑?s☻♥?▬??Uq3????=.v?}9↓?6H☼W?}┬??c?^?.?6.?♥}?‼?.?‼▲?↓i??2?▲???I.?FPlQ?~?♦??'
<d$`¶▲???-d?↑?s☻♥?▬??Uq3????=.v?}9↓?6H☼W?}┬??c?^?.?6.?♥}?‼?.?‼▲?↓i??2?▲???I.?FPlQ?~?♦??'
<d$`¶▲???-d?↑?s☻♥?▬??Uq3????=.v?}9↓?6H☼W?}┬??c?^?.?6.?♥}?‼?.?‼▲?↓i??2?▲???I.?FPlQ?~?♦??'

<cut>

| Warning | 1292 | Incorrect datetime value: 'A♥??WL???l?4?????9??BK?♀|???▲?D??♀z▲_?<6?3o????↑(H??l↕O
| Warning | 1292 | Incorrect datetime value: 'A♥??WL???l?4?????9??BK?♀|???▲?D??♀z▲_?<6?3o????↑(H??l↕O
| Warning | 1292 | Incorrect datetime value: 'A♥??WL???l?4?????9??BK?♀|???▲?D??♀z▲_?<6?3o????↑(H??l↕O
| Warning | 1292 | Incorrect datetime value: 'A♥??WL???l?4?????9??BK?♀|???▲?D??♀z▲_?<6?3o????↑(H??l↕O
+---------+------+-----------------------------------------------------------------------------------
64 rows in set (0.00 sec)

How to repeat:
will upload a testcase later

hm, there was garbage in the one column, caused the mangled output.