Fix issue esnet#1143 - make sure terminating error message is inside …

…the JSON output

src/iperf_client_api.c

@@ -659,9 +659,13 @@ iperf_run_client(struct iperf_test * test)
     return 0;
 
   cleanup_and_fail:
+    iperf_errexit(test, "error - %s", iperf_strerror(i_errno));
     iperf_client_end(test);
-    if (test->json_output)
-	iperf_json_finish(test);
+    if (test->json_output) {
+	if (iperf_json_finish(test) < 0)
+	    return -1;  // It is o.k. that error will be logged later outside the JSON output since its creation failed
+    }
     iflush(test);
-    return -1;
+    return 0;   // Return 0 and not -1 since all terminating function were done here.
+                // Also prevents error message logging outside the already closed JSON output.
 }