compat/win32/pthread.c: Fix a sparse warning

Sparse issues a 'Using plain integer as NULL pointer' warning when
initializing an pthread_t structure with an '{ 0 }' initializer.
The first field of the pthread_t structure has type HANDLE (void *),
so in order to suppress the warning, we replace the initializer
expression with '{ NULL }'.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Ramsay Jones

compat/win32/pthread.c

@@ -52,7 +52,7 @@ int win32_pthread_join(pthread_t *thread, void **value_ptr)
 
 pthread_t pthread_self(void)
 {
-	pthread_t t = { 0 };
+	pthread_t t = { NULL };
 	t.tid = GetCurrentThreadId();
 	return t;
 }