patch 8.2.0101: crash when passing null object to ":echomsg"

Problem:    Crash when passing null object to ":echomsg".
Solution:   Check for NULL pointer. (Yasuhiro Matsumoto, closes #5460)

src/eval.c

@@ -5682,6 +5682,7 @@ tv_stringify(typval_T *varp, char_u *buf)
 {
     if (varp->v_type == VAR_LIST
 	    || varp->v_type == VAR_DICT
+	    || varp->v_type == VAR_BLOB
 	    || varp->v_type == VAR_FUNC
 	    || varp->v_type == VAR_PARTIAL
 	    || varp->v_type == VAR_FLOAT)
@@ -6057,6 +6058,12 @@ ex_execute(exarg_T *eap)
 		p = tv_get_string_buf(&rettv, buf);
 	    else
 		p = tv_stringify(&rettv, buf);
+	    if (p == NULL)
+	    {
+		clear_tv(&rettv);
+		ret = FAIL;
+		break;
+	    }
 	    len = (int)STRLEN(p);
 	    if (ga_grow(&ga, len + 2) == FAIL)
 	    {

src/testdir/test_messages.vim

@@ -268,3 +268,13 @@ func Test_message_more()
   call term_sendkeys(buf, ':q!')
   call StopVimInTerminal(buf)
 endfunc
+
+func Test_null()
+  echom test_null_list()
+  echom test_null_dict()
+  echom test_null_blob()
+  echom test_null_job()
+  echom test_null_string()
+  echom test_null_channel()
+  echom test_null_partial()
+endfunc

src/version.c

@@ -742,6 +742,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    101,
 /**/
     100,
 /**/