Replace corrupts serialized data if the search and replace are of different length

To reproduce:
```
php -r 'echo serialize("aaaaabbbbbbbbbbaaaaa"), PHP_EOL;' > test.txt
cat test.txt | ./go-search-replace bbbbbbbbbb ccccc
cat test.txt | ./go-search-replace bbbbbbbbbb ccccccccccccccc
```

Expected result:
```
s:15:"aaaaacccccaaaaa";
s:25:"aaaaacccccccccccccccaaaaa";
```

Actual result:
```
s:20:"aaaaacccccaaaaa";
s:20:"aaaaacccccccccccccccaaaaa";
```
