fixes compiler error with Clang 5.0 on OSX Mavericks

- error message was:
```
In file included from /users/behrens/git/eudaq/./main/include/eudaq/Event.hh:11:
/users/behrens/git/eudaq/./main/include/eudaq/Serializer.hh:57:13: error: no viable overloaded '>>='
          t >>= 8;
```
fixed bug in serializer routine read_int

- created by changes of commit 471f33c which fixed length of the temporary variable to ensure safe bit-shifting but still returned data defined by the width of the (now fixed) variable instead of the width of the template variable