[BUG] hdr_idx might be left uninitialized in some cases

When a request is invalid during RQ_BEFORE AND the debug mode is active,
the hdr_idx might be used uninitialized. Let's initialize it right after
the accept() for now.
diff --git a/src/client.c b/src/client.c
index c72f7e4..e4babba 100644
--- a/src/client.c
+++ b/src/client.c
@@ -252,6 +252,7 @@
 				pool_free(session, s);
 				return 0;
 			}
+			hdr_idx_init(&hreq->hdr_idx);
 		}
 
 		if ((p->mode == PR_MODE_TCP || p->mode == PR_MODE_HTTP)