print_tag_downloads(): Free ref variable

Make sure the ref variable is freed if we build a
"$basename-$version"-style ref.

This fixes following memory leak seen with "PATH_INFO=/cgit/refs/":

    ==8784== 323 bytes in 29 blocks are definitely lost in loss record 41 of 53
    ==8784==    at 0x4C2C04B: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
    ==8784==    by 0x56F2DF1: strdup (in /usr/lib/libc-2.17.so)
    ==8784==    by 0x46CA28: xstrdup (wrapper.c:35)
    ==8784==    by 0x410DA6: print_tag_downloads (ui-refs.c:115)
    ==8784==    by 0x410F02: print_tag (ui-refs.c:141)
    ==8784==    by 0x41128B: cgit_print_tags (ui-refs.c:230)
    ==8784==    by 0x41134D: cgit_print_refs (ui-refs.c:250)
    ==8784==    by 0x407C85: refs_fn (cmd.c:105)
    ==8784==    by 0x405DDF: process_request (cgit.c:566)
    ==8784==    by 0x407490: cache_process (cache.c:322)
    ==8784==    by 0x406C18: main (cgit.c:864)

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>

ui-refs.c

@@ -103,6 +103,7 @@ static void print_tag_downloads(const struct cgit_repo *repo, const char *ref)
 	const struct cgit_snapshot_format* f;
     	char *filename;
 	const char *basename;
+	int free_ref = 0;
 
 	if (!ref || strlen(ref) < 2)
 		return;
@@ -111,8 +112,10 @@ static void print_tag_downloads(const struct cgit_repo *repo, const char *ref)
 	if (prefixcmp(ref, basename) != 0) {
 		if ((ref[0] == 'v' || ref[0] == 'V') && isdigit(ref[1]))
 			ref++;
-		if (isdigit(ref[0]))
+		if (isdigit(ref[0])) {
 			ref = xstrdup(fmt("%s-%s", basename, ref));
+			free_ref = 1;
+		}
 	}
 
 	for (f = cgit_snapshot_formats; f->suffix; f++) {
@@ -122,6 +125,9 @@ static void print_tag_downloads(const struct cgit_repo *repo, const char *ref)
 		cgit_snapshot_link(filename, NULL, NULL, NULL, NULL, filename);
 		html("&nbsp;&nbsp;");
 	}
+
+	if (free_ref)
+		free((char *)ref);
 }
 static int print_tag(struct refinfo *ref)
 {