patch 9.0.1444: crash when passing NULL to setcmdline()

Problem:    Crash when passing NULL to setcmdline(). (Andreas Louv)
Solution:   Use tv_get_string() instead of using v_string directly.
            (closes #12231, closes #12227)

src/ex_getln.c

@@ -4294,7 +4294,8 @@ f_setcmdline(typval_T *argvars, typval_T *rettv)
 	}
     }
 
-    rettv->vval.v_number = set_cmdline_str(argvars[0].vval.v_string, pos);
+    // Use tv_get_string() to handle a NULL string like an empty string.
+    rettv->vval.v_number = set_cmdline_str(tv_get_string(&argvars[0]), pos);
 }
 
 /*

src/testdir/test_cmdline.vim

@@ -3393,6 +3393,14 @@ endfunc
 
 func Test_setcmdline()
   func SetText(text, pos)
+    call assert_equal(0, setcmdline(test_null_string()))
+    call assert_equal('', getcmdline())
+    call assert_equal(1, getcmdpos())
+
+    call assert_equal(0, setcmdline(''[: -1]))
+    call assert_equal('', getcmdline())
+    call assert_equal(1, getcmdpos())
+
     autocmd CmdlineChanged * let g:cmdtype = expand('<afile>')
     call assert_equal(0, setcmdline(a:text))
     call assert_equal(a:text, getcmdline())

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1444,
 /**/
     1443,
 /**/