BUG/MINOR: sample: check alloc_trash_chunk return value in concat()

BUG/MINOR: sample: check alloc_trash_chunk return value in concat()

like it is done in other places, check the return value of
`alloc_trash_chunk` before using it. This was detected by coverity.

this patch fixes commit 591fc3a330005c289b4705fe4cb37c4eec9f9eed
("BUG/MINOR: sample: fix concat() converter's corruption with non-string
variables"
As a consequence, this patch should be backported as far as 2.0

this should fix github issue #1039

Signed-off-by: William Dauchy <wdauchy@gmail.com>
(cherry picked from commit e997010accdb17e70b628bc1e7c2810487d279c0)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>