libc: Check for NULL pointers in free()

POSIX says that the free() function should simply do nothing if a NULL
pointer argument has been specified. So let's be a little bit more
compliant in our libc and add a NULL pointer check here, too.

Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>

lib/libc/stdlib/free.c

@@ -19,8 +19,10 @@ free(void *ptr)
 {
 	struct chunk *header;
 
+	if (!ptr)
+		return;
+
 	header = (struct chunk *) ptr;
 	header--;
 	header->inuse = 0;
-
 }