find_default_branch(): Free refmatch after usage

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

    ==13408== 7 bytes in 1 blocks are definitely lost in loss record 4 of 52
    ==13408==    at 0x4C2C04B: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
    ==13408==    by 0x56F2DF1: strdup (in /usr/lib/libc-2.17.so)
    ==13408==    by 0x46CA78: xstrdup (wrapper.c:35)
    ==13408==    by 0x405840: find_current_ref (cgit.c:426)
    ==13408==    by 0x44BE5A: do_one_ref (refs.c:527)
    ==13408==    by 0x44D3E0: do_for_each_ref_in_dir (refs.c:553)
    ==13408==    by 0x44D85A: do_for_each_ref (refs.c:1298)
    ==13408==    by 0x405889: find_default_branch (cgit.c:438)
    ==13408==    by 0x405AC4: prepare_repo_cmd (cgit.c:490)
    ==13408==    by 0x405D97: process_request (cgit.c:557)
    ==13408==    by 0x407490: cache_process (cache.c:322)
    ==13408==    by 0x406C18: main (cgit.c:864)

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

cgit.c

@@ -427,6 +427,12 @@ static int find_current_ref(const char *refname, const unsigned char *sha1,
 	return info->match;
 }
 
+static void free_refmatch_inner(struct refmatch *info)
+{
+	if (info->first_ref)
+		free(info->first_ref);
+}
+
 static char *find_default_branch(struct cgit_repo *repo)
 {
 	struct refmatch info;
@@ -442,6 +448,8 @@ static char *find_default_branch(struct cgit_repo *repo)
 		ref = info.first_ref;
 	if (ref)
 		ref = xstrdup(ref);
+	free_refmatch_inner(&info);
+
 	return ref;
 }