BUG/MINOR: 51d: Get the request channel to call CHECK_HTTP_MESSAGE_FIRST()

Since the commit 89dc49935 ("BUG/MAJOR: http_fetch: Get the channel depending on
the keyword used"), the right channel must be passed as argument when the macro
CHECK_HTTP_MESSAGE_FIRST is called.

This patch must be backported to 1.9.
diff --git a/src/51d.c b/src/51d.c
index c87525d..a79b4d7 100644
--- a/src/51d.c
+++ b/src/51d.c
@@ -412,7 +412,7 @@
 	 * Data type has to be reset to ensure the string output is processed
 	 * correctly.
 	 */
-	CHECK_HTTP_MESSAGE_FIRST();
+	CHECK_HTTP_MESSAGE_FIRST((smp->strm ? &smp->strm->req : NULL));
 	smp->data.type = SMP_T_STR;
 
 	/* Flags the sample to show it uses constant memory*/