BUG/MEDIUM: 51d: possible incorrect operations on smp->data.str.str

BUG/MEDIUM: 51d: possible incorrect operations on smp->data.str.str

In src/51d.c, the function _51d_conv(), a final '\0' is added into
smp->data.str.str, which can cause a problem if the SMP_F_CONST flag is
set in smp->flags or if smp->data.str.size is not available.

This patch adds a check on smp->flags and smp->data.str.size, and copies
the smp->data.str.str to another buffer by using smp_dup(). If necessary,
the "const" flag is set after device detection. Also, this patch removes
the unnecessary call to chunk_reset() on temp argument.