CLEANUP: Use isttest(const struct ist) whenever possible

Refactoring performed with the following Coccinelle patch:

    @@
    struct ist i;
    @@

    - i.ptr != NULL
    + isttest(i)
diff --git a/src/hpack-dec.c b/src/hpack-dec.c
index debd611..4fa9bfd 100644
--- a/src/hpack-dec.c
+++ b/src/hpack-dec.c
@@ -118,7 +118,7 @@
 
 	out.len = in.len;
 	out.ptr = chunk_newstr(store);
-	if (unlikely(!out.ptr))
+	if (unlikely(!isttest(out)))
 		return out;
 
 	if (unlikely(store->data + out.len > store->size)) {