CLEANUP: compression: do not test for buffer before calling b_alloc()

Now we know the function is idempotent, we don't need to run the
preliminary test anymore.
diff --git a/src/flt_http_comp.c b/src/flt_http_comp.c
index de61cac..c9eb16b 100644
--- a/src/flt_http_comp.c
+++ b/src/flt_http_comp.c
@@ -66,9 +66,9 @@
 static int
 comp_flt_init_per_thread(struct proxy *px, struct flt_conf *fconf)
 {
-	if (!tmpbuf.size && b_alloc(&tmpbuf) == NULL)
+	if (b_alloc(&tmpbuf) == NULL)
 		return -1;
-	if (!zbuf.size && b_alloc(&zbuf) == NULL)
+	if (b_alloc(&zbuf) == NULL)
 		return -1;
 	return 0;
 }