BUG/MINOR: qpack: do not consider empty enc/dec stream as error

When parsing QPACK encoder/decoder streams, h3_decode_qcs() displays an
error trace if they are empty. Change the return code used in QPACK code
to avoid this trace.

To uniformize with MUX/H3 code, 0 is now used to indicate success.

Beyond this spurious error trace, this bug has no impact.
diff --git a/src/h3.c b/src/h3.c
index 831dc19..1b8e378 100644
--- a/src/h3.c
+++ b/src/h3.c
@@ -238,11 +238,11 @@
 
 	switch (h3s->type) {
 	case H3S_T_QPACK_DEC:
-		if (!qpack_decode_dec(qcs, NULL))
+		if (qpack_decode_dec(qcs, NULL))
 			return 1;
 		break;
 	case H3S_T_QPACK_ENC:
-		if (!qpack_decode_enc(qcs, NULL))
+		if (qpack_decode_enc(qcs, NULL))
 			return 1;
 		break;
 	case H3S_T_UNKNOWN:
diff --git a/src/qpack-dec.c b/src/qpack-dec.c
index c5e42a0..3ca2eda 100644
--- a/src/qpack-dec.c
+++ b/src/qpack-dec.c
@@ -93,7 +93,10 @@
 	return 0;
 }
 
-/* Decode an encoder stream */
+/* Decode an encoder stream.
+ *
+ * Returns 0 on success else non-zero.
+ */
 int qpack_decode_enc(struct qcs *qcs, void *ctx)
 {
 	size_t len;
@@ -123,10 +126,13 @@
 		/* Set dynamic table capacity */
 	}
 
-	return 1;
+	return 0;
 }
 
-/* Decode an decoder stream */
+/* Decode an decoder stream.
+ *
+ * Returns 0 on success else non-zero.
+ */
 int qpack_decode_dec(struct qcs *qcs, void *ctx)
 {
 	size_t len;
@@ -153,7 +159,7 @@
 		/* Stream cancellation */
 	}
 
-	return 1;
+	return 0;
 }
 
 /* Decode a field section prefix made of <enc_ric> and <db> two varints.