patch 8.2.3380: crash when using NULL string for funcref()

Problem:    Crash when using NULL string for funcref().
Solution:   Check for NULL argument. (issue #8260)

src/evalfunc.c

@@ -3932,6 +3932,11 @@ common_function(typval_T *argvars, typval_T *rettv, int is_funcref)
 	s = tv_get_string(&argvars[0]);
 	use_string = TRUE;
     }
+    if (s == NULL)
+    {
+	semsg(_(e_invarg2), "NULL");
+	return;
+    }
 
     if ((use_string && vim_strchr(s, AUTOLOAD_CHAR) == NULL) || is_funcref)
     {

src/testdir/test_expr.vim

@@ -501,6 +501,7 @@ func Test_funcref()
   let OneByRef = funcref("One", repeat(["foo"], 20))
   call assert_fails('let OneByRef = funcref("One", repeat(["foo"], 21))', 'E118:')
   call assert_fails('echo function("min") =~ function("min")', 'E694:')
+  call assert_fails('echo test_null_function()->funcref()', 'E475: Invalid argument: NULL')
 endfunc
 
 func Test_setmatches()

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3380,
 /**/
     3379,
 /**/