MINOR: spoe: Set the argument chunk size to 0 when SPOE variables are checked

MINOR: spoe: Set the argument chunk size to 0 when SPOE variables are checked

When SPOE variables are registered during HAProxy startup, the argument used to
call the function vars_check_arg() uses the trash area. To be sure it is never
released by the callee function, the size of the internal chunk (arg.data.str)
is set to 0. It is important to do so because, to fix a memory leak, this buffer
must be released by the function vars_check_arg().

This patch must be backported to 1.9.