Make sure delete_all function pointer is NULL

The peril of not using calloc mean delete_all is not initialized until
we explcitly set it. Explicitly set delete_all to NULL to avoid that.

Change-Id: Ic370453e6142c6d1b2566df9844b4fac4bc53042
diff --git a/softkeymaster/keymaster_openssl.cpp b/softkeymaster/keymaster_openssl.cpp
index a5ba684..5a7327b 100644
--- a/softkeymaster/keymaster_openssl.cpp
+++ b/softkeymaster/keymaster_openssl.cpp
@@ -486,6 +486,7 @@
     dev->import_keypair = openssl_import_keypair;
     dev->get_keypair_public = openssl_get_keypair_public;
     dev->delete_keypair = NULL;
+    dev->delete_all = NULL;
     dev->sign_data = openssl_sign_data;
     dev->verify_data = openssl_verify_data;