zookeeper: initialize a variable

If the loop on line 132 doesn't iterate at least once, the function would
return the "sk" variable uninitialized.

This fixes the following build error:

```
cc1: warnings being treated as errors
zookeeper.c: In function 'zookeeper_read':
zookeeper.c:107: warning: 'sk' may be used uninitialized in this function
make[3]: *** [zookeeper.lo] Error 1
```

(caught by gcc version 4.1.2 on EL5)