patch 8.2.1773: crash when calling mapset() with a list as first argument

Problem:    Crash when calling mapset() with a list as first argument.
Solution:   Check for NULL. (closes #7040)

src/map.c

@@ -2304,6 +2304,8 @@ f_mapset(typval_T *argvars, typval_T *rettv UNUSED)
     char_u	*arg;
 
     which = tv_get_string_buf_chk(&argvars[0], buf);
+    if (which == NULL)
+	return;
     mode = get_map_mode(&which, 0);
     is_abbr = (int)tv_get_number(&argvars[1]);
 

src/testdir/test_maparg.vim

@@ -238,6 +238,8 @@ func Test_mapset()
   iunmap K
   let &cpo = cpo_save
   bwipe!
+
+  call assert_fails('call mapset([], v:false, {})', 'E730:')
 endfunc
 
 func Check_ctrlb_map(d, check_alt)

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1773,
 /**/
     1772,
 /**/