BUG/MINOR: sample: fix concat() converter's corruption with non-string variables

BUG/MINOR: sample: fix concat() converter's corruption with non-string variables

Patrick Hemmer reported that calling concat() with an integer variable
causes a %00 to appear at the beginning of the output. Looking at the
code, it's not surprising. The function uses get_trash_chunk() to get
one of the trashes, but can call casting functions which will also use
their trash in turn and will cycle back to ours, causing the trash to
be overwritten before being assigned to a sample.

By allocating the trash from a pool using alloc_trash_chunk(), we can
avoid this. However we must free it so the trash's contents must be
moved to a permanent trash buffer before returning. This is what's
achieved using smp_dup().

This should be backported as far as 2.0.