fetch-pack: do not use lockfile structure on stack.

They are used in atexit() for clean-up, and you will be
accessing unallocated memory at that point.

See 31f584c2 for the fix for a similar problem.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

fetch-pack.c

@@ -625,6 +625,8 @@ static int remove_duplicates(int nr_heads, char **heads)
 	return dst;
 }
 
+static struct lock_file lock;
+
 int main(int argc, char **argv)
 {
 	int i, ret, nr_heads;
@@ -632,7 +634,6 @@ int main(int argc, char **argv)
 	int fd[2];
 	pid_t pid;
 	struct stat st;
-	struct lock_file lock;
 
 	setup_git_directory();