os: fix off-by-one error in ShellEscapeWriter (#9842)

I am truly not sure why the tests never caught this, but I just fell for
the oldest trick in the book

Fixes a crash on GTK when a file is dragged then dropped into the
terminal - should not impact 1.2 as this is a casualty of the Zig 0.15
port

src/os/main.zig

@@ -69,6 +69,7 @@ test {
     _ = i18n;
     _ = path;
     _ = uri;
+    _ = shell;
 
     if (comptime builtin.os.tag == .linux) {
         _ = kernel_info;

src/os/shell.zig

@@ -5,8 +5,6 @@ const Writer = std.Io.Writer;
 /// Writer that escapes characters that shells treat specially to reduce the
 /// risk of injection attacks or other such weirdness. Specifically excludes
 /// linefeeds so that they can be used to delineate lists of file paths.
-///
-/// T should be a Zig type that follows the `std.Io.Writer` interface.
 pub const ShellEscapeWriter = struct {
     writer: Writer,
     child: *Writer,
@@ -33,7 +31,7 @@ pub const ShellEscapeWriter = struct {
         var count: usize = 0;
         for (data[0 .. data.len - 1]) |chunk| try self.writeEscaped(chunk, &count);
 
-        for (0..splat) |_| try self.writeEscaped(data[data.len], &count);
+        for (0..splat) |_| try self.writeEscaped(data[data.len - 1], &count);
         return count;
     }
 
@@ -67,7 +65,7 @@ pub const ShellEscapeWriter = struct {
 test "shell escape 1" {
     var buf: [128]u8 = undefined;
     var writer: std.Io.Writer = .fixed(&buf);
-    var shell: ShellEscapeWriter = .{ .child_writer = &writer };
+    var shell: ShellEscapeWriter = .init(&writer);
     try shell.writer.writeAll("abc");
     try testing.expectEqualStrings("abc", writer.buffered());
 }
@@ -75,7 +73,7 @@ test "shell escape 1" {
 test "shell escape 2" {
     var buf: [128]u8 = undefined;
     var writer: std.Io.Writer = .fixed(&buf);
-    var shell: ShellEscapeWriter = .{ .child_writer = &writer };
+    var shell: ShellEscapeWriter = .init(&writer);
     try shell.writer.writeAll("a c");
     try testing.expectEqualStrings("a\\ c", writer.buffered());
 }
@@ -83,7 +81,7 @@ test "shell escape 2" {
 test "shell escape 3" {
     var buf: [128]u8 = undefined;
     var writer: std.Io.Writer = .fixed(&buf);
-    var shell: ShellEscapeWriter = .{ .child_writer = &writer };
+    var shell: ShellEscapeWriter = .init(&writer);
     try shell.writer.writeAll("a?c");
     try testing.expectEqualStrings("a\\?c", writer.buffered());
 }
@@ -91,7 +89,7 @@ test "shell escape 3" {
 test "shell escape 4" {
     var buf: [128]u8 = undefined;
     var writer: std.Io.Writer = .fixed(&buf);
-    var shell: ShellEscapeWriter = .{ .child_writer = &writer };
+    var shell: ShellEscapeWriter = .init(&writer);
     try shell.writer.writeAll("a\\c");
     try testing.expectEqualStrings("a\\\\c", writer.buffered());
 }
@@ -99,7 +97,7 @@ test "shell escape 4" {
 test "shell escape 5" {
     var buf: [128]u8 = undefined;
     var writer: std.Io.Writer = .fixed(&buf);
-    var shell: ShellEscapeWriter = .{ .child_writer = &writer };
+    var shell: ShellEscapeWriter = .init(&writer);
     try shell.writer.writeAll("a|c");
     try testing.expectEqualStrings("a\\|c", writer.buffered());
 }
@@ -107,7 +105,7 @@ test "shell escape 5" {
 test "shell escape 6" {
     var buf: [128]u8 = undefined;
     var writer: std.Io.Writer = .fixed(&buf);
-    var shell: ShellEscapeWriter = .{ .child_writer = &writer };
+    var shell: ShellEscapeWriter = .init(&writer);
     try shell.writer.writeAll("a\"c");
     try testing.expectEqualStrings("a\\\"c", writer.buffered());
 }
@@ -115,7 +113,7 @@ test "shell escape 6" {
 test "shell escape 7" {
     var buf: [128]u8 = undefined;
     var writer: std.Io.Writer = .fixed(&buf);
-    var shell: ShellEscapeWriter = .{ .child_writer = &writer };
+    var shell: ShellEscapeWriter = .init(&writer);
     try shell.writer.writeAll("a(1)");
     try testing.expectEqualStrings("a\\(1\\)", writer.buffered());
 }