BUG/MINOR: map: fix map_regm with backref

Due to a cascade of get_trash_chunk calls the sample is
corrupted when we want to read it.

The fix consist to use a temporary chunk to copy the sample
value and use it.

[wt: for 1.8 and older, a backport was successfully tested here :
 https://www.mail-archive.com/haproxy@formilux.org/msg30694.html]
1 file changed