MINOR: spoe: Handle NOTIFY frames cancellation using ABORT bit in ACK frames

If an agent want to abort the processing a fragmented NOTIFY frame before
receiving all fragments, it can send an ACK frame at any time with ABORT bit set
(and of course, the FIN bit too).

Beside this change, SPOE_FRM_ERR_FRAMEID_NOTFOUND error flag has been added. It
is set when a unknown ACK frame is received.
diff --git a/contrib/spoa_example/spoa.c b/contrib/spoa_example/spoa.c
index 8e234b5..cbea24d 100644
--- a/contrib/spoa_example/spoa.c
+++ b/contrib/spoa_example/spoa.c
@@ -100,6 +100,7 @@
 	SPOE_FRM_ERR_BAD_FRAME_SIZE,
 	SPOE_FRM_ERR_FRAG_NOT_SUPPORTED,
 	SPOE_FRM_ERR_INTERLACED_FRAMES,
+	SPOE_FRM_ERR_FRAMEID_NOTFOUND,
 	SPOE_FRM_ERR_RES,
 	SPOE_FRM_ERR_UNKNOWN = 99,
 	SPOE_FRM_ERRS,
@@ -273,6 +274,7 @@
 	[SPOE_FRM_ERR_BAD_FRAME_SIZE]     = "max-frame-size too big or too small",
 	[SPOE_FRM_ERR_FRAG_NOT_SUPPORTED] = "fragmentation not supported",
 	[SPOE_FRM_ERR_INTERLACED_FRAMES]  = "invalid interlaced frames",
+	[SPOE_FRM_ERR_FRAMEID_NOTFOUND]   = "frame-id not found",
 	[SPOE_FRM_ERR_RES]                = "resource allocation error",
 	[SPOE_FRM_ERR_UNKNOWN]            = "an unknown error occurred",
 };
@@ -1672,7 +1674,7 @@
 				    spoe_frm_err_reasons[client->status_code]);
 				goto disconnect;
 			}
-			if (n == 0) {
+			else if (n == 0) {
 				LOG(client->worker, "Ignore invalid/unknown/aborted frame");
 				goto ignore_frame;
 			}