dload: dont forget to initialize open_mode

That's a funny one, building with optimization levels (with both gcc and
clang) caused open_mode to always be set to "ab", which worked.

This was spotted both with clang-analyzer, and by Jakob who reported a
segfault as he was using an un-optimized build.

Signed-off-by: Xavier Chantry <chantry.xavier@gmail.com>
Signed-off-by: Dan McGee <dan@archlinux.org>

lib/libalpm/dload.c

@@ -159,7 +159,8 @@ static int curl_download_internal(const char *url, const char *localpath,
 {
 	int ret = -1;
 	FILE *localf = NULL;
-	const char *open_mode, *useragent;
+	const char *useragent;
+	const char *open_mode = "wb";
 	char *destfile, *tempfile;
 	char hostname[256]; /* RFC1123 states applications should support this length */
 	struct stat st;