Unify freeing of target->zck_dl to a single place

Apart from these two places the `zck_dl` is leaking through other code
paths. Handle it and stuff it contains in a single place.

librepo/downloader.c

@@ -1357,8 +1357,6 @@ check_zck(LrTarget *target, GError **err)
 
         if(cks_good == 1) {  // All checksums good
             g_debug("%s: File is complete", __func__);
-            if(target->target->zck_dl)
-                zck_dl_free(&(target->target->zck_dl));
             target->zck_state = LR_ZCK_DL_FINISHED;
             return TRUE;
         }
@@ -1378,8 +1376,6 @@ check_zck(LrTarget *target, GError **err)
         }
 
         if(cks_good == 1) {  // All checksums good
-            if(target->target->zck_dl)
-                zck_dl_free(&(target->target->zck_dl));
             target->zck_state = LR_ZCK_DL_FINISHED;
             return TRUE;
         }

librepo/downloadtarget.c

@@ -134,6 +134,26 @@ lr_downloadtarget_free(LrDownloadTarget *target)
     if (!target)
         return;
 
+    #ifdef WITH_ZCHUNK
+    if (target->zck_dl) {
+        zckCtx *old_zck = zck_dl_get_zck(target->zck_dl);
+        if (old_zck) {
+            zck_free(&old_zck);
+        }
+
+        zckRange *old_range = zck_dl_get_range(target->zck_dl);
+        if(old_range) {
+            zck_range_free(&old_range);
+        }
+
+        zck_dl_free(&target->zck_dl);
+    }
+    #endif /* WITH_ZCHUNK */
+
+    if(target->range) {
+        free(target->range);
+    }
+
     g_slist_free_full(target->checksums,
                       (GDestroyNotify) lr_downloadtargetchecksum_free);
     g_string_chunk_free(target->chunk);