reset args after init, which now zeroes them

Fixes #3
was actually caused by the fix for #2 (and I failed to recognize this)

src/test/test_format.c

@@ -1644,6 +1644,8 @@ static const char
     reset_hdrs();
     add_hdr(SLT_Timestamp, "Resp", 0);
     init_tx_arg(&tx, node, nptr, &args);
+    args.fld = -1;
+    args.hdr_idx = -1;
     init_hdr_recs(&tx, SLT_Timestamp);
     init_rec_chunk(SLT_Timestamp, &rec, &chunk);
     set_record_data(&rec, &chunk, TS_RESP_PAYLOAD, SLT_Timestamp);