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);
     }

src/json.c

@@ -51,6 +51,7 @@ json_encode(typval_T *val, int options)
     /* Store bytes in the growarray. */
     ga_init2(&ga, 1, 4000);
     json_encode_gap(&ga, val, options);
+    ga_append(&ga, NUL);
     return ga.ga_data;
 }
 
@@ -82,6 +83,7 @@ json_encode_nr_expr(int nr, typval_T *val, int options)
     if (json_encode_gap(&ga, &listtv, options) == OK && (options & JSON_NL))
 	ga_append(&ga, '\n');
     list_unref(listtv.vval.v_list);
+    ga_append(&ga, NUL);
     return ga.ga_data;
 }
 #endif

src/testdir/test_json.vim

@@ -289,3 +289,10 @@ func Test_js_decode()
 
   call assert_equal(s:varl5, js_decode(s:jsl5))
 endfunc
+
+func Test_json_encode_long()
+  " The growarray uses a grow size of 4000, check that a result that is exactly
+  " 4000 bytes long is not missing the final NUL.
+  let json = json_encode([repeat('a', 3996)])
+  call assert_equal(4000, len(json))
+endfunc

src/version.c

@@ -771,6 +771,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1158,
 /**/
     1157,
 /**/