BUG/MEDIUM: compression: fix the output type of the compressor name

smp_fetch_res_comp_algo() returns the name of the compression algorithm
in use. The output type is set to SMP_T_STR instead of SMP_T_CSTR, which
causes any transformation to be operated without a cast. Fortunately,
the current converters do not overwrite a zero-sized area, so the result
is an empty string. Fix this to have SMP_T_CSTR instead so that the cast
is always performed using a copy before any transformation is done.