szip: fix panic on empty files (#24335)

Author: JalonSolov

vlib/compress/szip/szip.c.v

@@ -183,7 +183,7 @@ pub fn (mut zentry Zip) crc32() u32 {
 
 // write_entry compresses an input buffer for the current zip entry.
 pub fn (mut zentry Zip) write_entry(data []u8) ! {
-	if int(data[0] & 0xff) == -1 {
+	if data.len > 0 && int(data[0] & 0xff) == -1 {
 		return error('szip: cannot write entry')
 	}
 	res := C.zip_entry_write(zentry, data.data, data.len)

vlib/compress/szip/szip_test.v

@@ -170,3 +170,11 @@ fn test_zip_folder_omit_empty_directories() {
 	assert (os.read_file(fpath5)!) == '5'
 	assert (os.read_file(fpath6)!) == '6'
 }
+
+fn test_zip_folder_empty_file() {
+	cleanup()
+	os.mkdir_all(test_path)!
+	os.write_file('${test_path}/test.txt', '')! // Empty file
+	szip.zip_folder(test_path, test_dir_zip)!
+	assert os.exists(test_dir_zip)
+}