BUG/MINOR: compression/htx: Don't add the last block of data if it is empty

In HTX, when the compression filter analyze the EOM, it flushes the compression
context and add the last block of compressed data. But, this block can be
empty. In this case, we must ignore it.
diff --git a/src/flt_http_comp.c b/src/flt_http_comp.c
index ce0dba1..380366b 100644
--- a/src/flt_http_comp.c
+++ b/src/flt_http_comp.c
@@ -255,10 +255,12 @@
 					}
 					if (htx_compression_buffer_end(st, &trash, 1) < 0)
 						goto error;
-					blk = htx_add_data_before(htx, blk, ist2(b_head(&trash), b_data(&trash)));
-					if (!blk)
-						goto error;
-					to_forward += b_data(&trash);
+					if (b_data(&trash)) {
+						blk = htx_add_data_before(htx, blk, ist2(b_head(&trash), b_data(&trash)));
+						if (!blk)
+							goto error;
+						to_forward += b_data(&trash);
+					}
 					msg->flags &= ~HTTP_MSGF_COMPRESSING;
 					/* We let the mux add last empty chunk and empty trailers */
 				}