Fix database schema

Replace "ON SET NULL" with "ON DELETE SET NULL". Fixes a regression
introduced in commit fb7bde3a

 (Add support for anonymous comments,
2014-02-04).
Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>

schema/aur-schema.sql

@@ -167,7 +167,7 @@ CREATE TABLE PackageComments (
 	PRIMARY KEY (ID),
 	INDEX (UsersID),
 	INDEX (PackageID),
-	FOREIGN KEY (UsersID) REFERENCES Users(ID) ON SET NULL,
+	FOREIGN KEY (UsersID) REFERENCES Users(ID) ON DELETE SET NULL,
 	FOREIGN KEY (DelUsersID) REFERENCES Users(ID) ON DELETE CASCADE,
 	FOREIGN KEY (PackageID) REFERENCES Packages(ID) ON DELETE CASCADE
 ) ENGINE = InnoDB;