lib/libalpm/dload.c: Add ASSERT() to alpm_fetch_pkgurl()

Return with ALPM_ERR_WRONG_ARGS instead of causing a potential segfault
if alpm_fetch_pkgurl() is invoked with a NULL URL.

Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>
Signed-off-by: Dan McGee <dan@archlinux.org>

lib/libalpm/dload.c

@@ -450,6 +450,7 @@ char SYMEXPORT *alpm_fetch_pkgurl(alpm_handle_t *handle, const char *url)
 	int ret;
 
 	CHECK_HANDLE(handle, return NULL);
+	ASSERT(url, RET_ERR(handle, ALPM_ERR_WRONG_ARGS, NULL));
 
 	/* find a valid cache dir to download to */
 	cachedir = _alpm_filecache_setup(handle);