patch 8.1.1158: json encoded string is sometimes missing the final NUL

Problem:    Json encoded string is sometimes missing the final NUL.
Solution:   Add the NUL.  Also for log messages.

src/channel.c

@@ -5877,6 +5877,7 @@ job_start(
 		ga_concat(&ga, (char_u *)"  ");
 	    ga_concat(&ga, (char_u *)argv[i]);
 	}
+	ga_append(&ga, NUL);
 	ch_log(NULL, "Starting job: %s", (char *)ga.ga_data);
 	ga_clear(&ga);
     }