Delete a trivially true check for a variable 'S' being null.

The exact same test guards entry into the loop in which this test
occurs, and there is nothing inside the loop that assigns to the
variable, so it has already been checked for null.

This was flagged by PVS-Studio as well, but the report is actually wrong
-- this is not a case where we dereference a variable prior to testing
it for null, this is a case where we have a redundant test for null
after we already performed the exact same test.

llvm-svn: 285983