mesa: Ensure gl_sync_object is fully initialized.

278372b47e added the uninitialized pointer
field gl_sync_object:Label. A free of this pointer, added in commit
6d8dd59cf5, resulted in a crash.

This patch fixes piglit ARB_sync regressions with swrast introduced by
6d8dd59cf5.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/mesa/main/syncobj.c

@@ -71,7 +71,7 @@
 static struct gl_sync_object *
 _mesa_new_sync_object(struct gl_context *ctx, GLenum type)
 {
-   struct gl_sync_object *s = MALLOC_STRUCT(gl_sync_object);
+   struct gl_sync_object *s = CALLOC_STRUCT(gl_sync_object);
    (void) ctx;
    (void) type;