Make fish_indent stop panicing on closed stdin

Prior to this commit, this code:

    fish_indent <&-

would panic as we would construct a File with a negative fd.
Check for a closed fd as other builtins do.

tests/checks/indent.fish

@@ -652,3 +652,7 @@ cat $tmpdir/indent_test.fish
 # See that the builtin can be redirected
 printf %s\n a b c | builtin fish_indent | grep b
 # CHECK: b
+
+# Regression test that fish_indent doesn't panic with closed stdin.
+fish_indent <&-
+# CHECKERR: fish_indent: stdin is closed