MINOR: stats: Add the status code STAT_STATUS_IVAL to handle invalid requests

This patch must be backported to 1.9 because a bug fix depends on it.
diff --git a/src/proto_http.c b/src/proto_http.c
index 4361d6d..0127e23 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -84,6 +84,7 @@
 	[STAT_STATUS_NONE] = "NONE",
 	[STAT_STATUS_PART] = "PART",
 	[STAT_STATUS_UNKN] = "UNKN",
+	[STAT_STATUS_IVAL] = "IVAL",
 };
 
 /* This function handles a server error at the stream interface level. The