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.
diff --git a/src/compression.c b/src/compression.c
index 27ba4b4..f42f771 100644
--- a/src/compression.c
+++ b/src/compression.c
@@ -628,7 +628,7 @@
 	if (!l4->comp_algo)
 		return 0;
 
-	smp->type = SMP_T_STR;
+	smp->type = SMP_T_CSTR;
 	smp->data.str.str = l4->comp_algo->name;
 	smp->data.str.len = l4->comp_algo->name_len;
 	return 1;