BUG/MINOR: hpack: fix debugging output of pseudo header names

When a pseudo header is used, name.ptr is NULL and we must replace it
with hpack_idx_to_name(). This only affects code built with DEBUG_HPACK.

To be backported to 1.8.
diff --git a/src/hpack-dec.c b/src/hpack-dec.c
index eadb6de..1a776bc 100644
--- a/src/hpack-dec.c
+++ b/src/hpack-dec.c
@@ -376,7 +376,7 @@
 		}
 
 		hpack_debug_printf("\e[1;34m%s\e[0m: ",
-		                   istpad(trash.str, name).ptr);
+		                   istpad(trash.str, name.ptr ? name : hpack_idx_to_name(dht, idx)).ptr);
 
 		hpack_debug_printf("\e[1;35m%s\e[0m [idx=%d, used=%d]\n",
 		                   istpad(trash.str, value).ptr,