Fix a memory leak in select_next_target()

If a next target URL was found (non-NULL full_url) and then a transfer was
canceled or an off-line mode was requested, full_url string was not freed and a
memory leaked.

Discovered with Covscan:

    16. librepo-1.18.0/librepo/downloader.c:891:13: alloc_fn: Storage is returned from allocation function "g_strdup_inline".
    17. librepo-1.18.0/librepo/downloader.c:891:13: var_assign: Assigning: "full_url" = storage returned from "g_strdup_inline(target->target->path)".
    22. librepo-1.18.0/librepo/downloader.c:919:9: noescape: Resource "full_url" is not freed or pointed-to in "lr_is_local_path".
    24. librepo-1.18.0/librepo/downloader.c:924:13: noescape: Assuming resource "full_url" is not freed or pointed-to as ellipsis argument to "g_debug".
    28. librepo-1.18.0/librepo/downloader.c:956:17: leaked_storage: Variable "full_url" going out of scope leaks the storage it points to.
    #   954|                               "and no local URL is available",
    #   955|                               target->target->path);
    #   956|->                 return FALSE;
    #   957|               }
    #   958|           }

    16. librepo-1.18.0/librepo/downloader.c:891:13: alloc_fn: Storage is returned from allocation function "g_strdup_inline".
    17. librepo-1.18.0/librepo/downloader.c:891:13: var_assign: Assigning: "full_url" = storage returned from "g_strdup_inline(target->target->path)".
    22. librepo-1.18.0/librepo/downloader.c:919:9: noescape: Resource "full_url" is not freed or pointed-to in "lr_is_local_path".
    24. librepo-1.18.0/librepo/downloader.c:924:13: noescape: Assuming resource "full_url" is not freed or pointed-to as ellipsis argument to "g_debug".
    27. librepo-1.18.0/librepo/downloader.c:946:21: leaked_storage: Variable "full_url" going out of scope leaks the storage it points to.
    #   944|                       g_set_error(err, LR_DOWNLOADER_ERROR, LRE_CBINTERRUPTED,
    #   945|                               "Interrupted by LR_CB_ERROR from end callback");
    #   946|->                     return FALSE;
    #   947|                   }
    #   948|               }

This patch fixes it.

The bug was introduced in 1.7.14 version
(08e4810 commit).

librepo/downloader.c

@@ -943,6 +943,7 @@ select_next_target(LrDownload *dd,
                             "from end callback", __func__);
                     g_set_error(err, LR_DOWNLOADER_ERROR, LRE_CBINTERRUPTED,
                             "Interrupted by LR_CB_ERROR from end callback");
+                    g_free(full_url);
                     return FALSE;
                 }
             }
@@ -953,6 +954,7 @@ select_next_target(LrDownload *dd,
                             "Cannot download %s: Offline mode is specified "
                             "and no local URL is available",
                             target->target->path);
+                g_free(full_url);
                 return FALSE;
             }
         }