compress.zlib: add overshoot test cast

Using example from [zigimg](https://github.com/zigimg/zigimg/pull/164) project.

lib/std/compress/zlib.zig

@@ -64,3 +64,38 @@ pub const store = struct {
         return deflate.store.compressor(.zlib, writer);
     }
 };
+
+test "should not overshoot" {
+    const std = @import("std");
+
+    // Compressed zlib data with extra 4 bytes at the end.
+    const data = [_]u8{
+        0x78, 0x9c, 0x73, 0xce, 0x2f, 0xa8, 0x2c, 0xca, 0x4c, 0xcf, 0x28, 0x51, 0x08, 0xcf, 0xcc, 0xc9,
+        0x49, 0xcd, 0x55, 0x28, 0x4b, 0xcc, 0x53, 0x08, 0x4e, 0xce, 0x48, 0xcc, 0xcc, 0xd6, 0x51, 0x08,
+        0xce, 0xcc, 0x4b, 0x4f, 0x2c, 0xc8, 0x2f, 0x4a, 0x55, 0x30, 0xb4, 0xb4, 0x34, 0xd5, 0xb5, 0x34,
+        0x03, 0x00, 0x8b, 0x61, 0x0f, 0xa4, 0x52, 0x5a, 0x94, 0x12,
+    };
+
+    var stream = std.io.fixedBufferStream(data[0..]);
+    const reader = stream.reader();
+
+    var dcp = decompressor(reader);
+    var out: [128]u8 = undefined;
+
+    // Decompress
+    var n = try dcp.reader().readAll(out[0..]);
+
+    // Expected decompressed data
+    try std.testing.expectEqual(46, n);
+    try std.testing.expectEqualStrings("Copyright Willem van Schaik, Singapore 1995-96", out[0..n]);
+
+    // Decompressor don't overshoot underlying reader.
+    // It is leaving it at the end of compressed data chunk.
+    try std.testing.expectEqual(data.len - 4, stream.getPos());
+    try std.testing.expectEqual(0, dcp.unreadBytes());
+
+    // 4 bytes after compressed chunk are available in reader.
+    n = try reader.readAll(out[0..]);
+    try std.testing.expectEqual(n, 4);
+    try std.testing.expectEqualSlices(u8, data[data.len - 4 .. data.len], out[0..n]);
+}