integrity: Fix sparse warnings in keyring_handler

Fix the following sparse warnings:

  CHECK   security/integrity/platform_certs/keyring_handler.c
security/integrity/platform_certs/keyring_handler.c:76:16: warning: Using plain integer as NULL pointer
security/integrity/platform_certs/keyring_handler.c:91:16: warning: Using plain integer as NULL pointer
security/integrity/platform_certs/keyring_handler.c:106:16: warning: Using plain integer as NULL pointer

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

Author: stefanberger

security/integrity/platform_certs/keyring_handler.c

@@ -73,7 +73,7 @@ __init efi_element_handler_t get_handler_for_db(const efi_guid_t *sig_type)
 {
 	if (efi_guidcmp(*sig_type, efi_cert_x509_guid) == 0)
 		return add_to_platform_keyring;
-	return 0;
+	return NULL;
 }
 
 /*
@@ -88,7 +88,7 @@ __init efi_element_handler_t get_handler_for_mok(const efi_guid_t *sig_type)
 		else
 			return add_to_platform_keyring;
 	}
-	return 0;
+	return NULL;
 }
 
 /*
@@ -103,5 +103,5 @@ __init efi_element_handler_t get_handler_for_dbx(const efi_guid_t *sig_type)
 		return uefi_blacklist_binary;
 	if (efi_guidcmp(*sig_type, efi_cert_x509_guid) == 0)
 		return uefi_revocation_list_x509;
-	return 0;
+	return NULL;
 }