CLEANUP: http: fix a build warning introduced by a recent fix

Cyril reported that recent commit 320ec2a ("BUG/MEDIUM: chunks: always
reject negative-length chunks") introduced a build warning because gcc
cannot guess that we can't fall into the case where the auth_method
chunk is not initialized.

This patch addresses it, though for the long term it would be best
if chunk_initlen() would always initialize the result.

This fix must be backported to 1.6 and 1.5 where the aforementionned
fix was already backported.
diff --git a/src/proto_http.c b/src/proto_http.c
index 8b5caad..b7654a6 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -1585,11 +1585,12 @@
 	h = ctx.line + ctx.val;
 
 	p = memchr(h, ' ', ctx.vlen);
-	if (!p || p == h)
+	len = p - h;
+	if (!p || len <= 0)
 		return 0;
 
-	chunk_initlen(&auth_method, h, 0, p-h);
-	chunk_initlen(&txn->auth.method_data, p+1, 0, ctx.vlen-(p-h)-1);
+	chunk_initlen(&auth_method, h, 0, len);
+	chunk_initlen(&txn->auth.method_data, p + 1, 0, ctx.vlen - len - 1);
 
 	if (!strncasecmp("Basic", auth_method.str, auth_method.len)) {