efi_selftest: avoid dereferencing NULL in tpl test

The task priority levels test uses two events one passes the
notification counter as context. The other passes NULL.
Both use the same notification function. So we need to check
for NULL here.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Alexander Graf <agraf@suse.de>
diff --git a/lib/efi_selftest/efi_selftest_tpl.c b/lib/efi_selftest/efi_selftest_tpl.c
index 0b78ee7..b8c0e70 100644
--- a/lib/efi_selftest/efi_selftest_tpl.c
+++ b/lib/efi_selftest/efi_selftest_tpl.c
@@ -26,7 +26,8 @@
 {
 	unsigned int *count = context;
 
-	++*count;
+	if (count)
+		++*count;
 }
 
 /*