BUG/MINOR: spoe: check snprintf() return value in spoe_set_var/spoe_unset_var

BUG/MINOR: spoe: check snprintf() return value in spoe_set_var/spoe_unset_var

snprintf() returns the length it would have written, not what actually
fit in varname[64]. A SET-VAR/UNSET-VAR name long enough to overflow
that buffer left len oversized, causing an out-of-bounds stack read in
the variable-name validation and hashing in vars_fill_desc().

Reject the action instead of proceeding on truncation: an oversized
name would otherwise be silently used under a different, truncated
name, which could let two distinct long names collide on the same
63-byte prefix.

This should be backported to all stable branches.

Reported-by: Aisle Research
Reported-by: Oyvind Albrigtsen <oalbrigt@redhat.com>