check return value of gzclose to detect unfinished gzip stream

geoipupdate does not report error even though GeoIP.dat on updates.maxmind.com is unfinished gzip.
Return value of gzclose() should be checked because only gzclose() can detect incomplete gzip stream.

http://www.zlib.net/manual.html#Gzip
> Note that gzread does not return -1 in the event of an incomplete gzip stream. 
> This error is deferred until gzclose(), which will return Z_BUF_ERROR if the last gzread ended 
> in the middle of a gzip stream. Alternatively, gzerror can be used before gzclose to detect this case. 

---

I tested my commit on the fake update site.

##### fake update site
$ ll
-rw-r--r-- 1 root root 1061834 Oct  6 14:20 GeoIP.dat.gz.orig
$ head -c 1000000 GeoIP.dat.gz.orig  > GeoIP.dat.gz.unfinished
$ ll
-rw-r--r-- 1 root root 1000000 Oct 21 10:46 GeoIP.dat.gz.unfinished
-rw-r--r-- 1 root root 1061834 Oct  6 14:20 GeoIP.dat.gz.orig

/etc/nginx/conf.d/default.conf
```
server {
    listen       80;
    server_name  geoip.localdomain;
    root /usr/share/nginx/html_geoip/;
    location / {
        try_files $uri /GeoIP.dat.gz.unfinished;
    }
}
```

##### geoipupdate client
/usr/local/etc/GeoIP.conf
```
UserId YOUR_USER_ID_HERE
LicenseKey YOUR_LICENSE_KEY_HERE
ProductIds GeoIP2-Country 106
Host geoip.localdomain
Protocol http
```
$ ll
$  geoipupdate
Gzip read error while closing from /usr/local/share/GeoIP/GeoIP.dat.gz
$ ll
-rw-r--r-- 1 root root 1000000 Oct 21 10:57 GeoIP.dat.gz