BUG/MINOR: spoe: Fix counters update when processing is interrupted

When the processing is interrupted, because of a typo, <nb_sending> was
incremented instead of decremented.
diff --git a/src/flt_spoe.c b/src/flt_spoe.c
index 7c019c4..40cdd69 100644
--- a/src/flt_spoe.c
+++ b/src/flt_spoe.c
@@ -2597,7 +2597,7 @@
 
 	if (!LIST_ISEMPTY(&ctx->list)) {
 		if (ctx->state == SPOE_CTX_ST_SENDING_MSGS)
-			HA_ATOMIC_ADD(&agent->counters.nb_sending, 1);
+			HA_ATOMIC_SUB(&agent->counters.nb_sending, 1);
 		else
 			HA_ATOMIC_SUB(&agent->counters.nb_waiting, 1);