Fix a memory leak in select_next_target()

[ppisar]

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 (08e4810fcdd753ce4728bd88b252f7b3d34b2cdb commit).

[kontura]

LGTM.