efi_selftests: fix protocol repeated selftesting

efi_selftests: fix protocol repeated selftesting

Running the protocols  selftest more than one times fails with

=> setenv efi_selftest 'manage protocols' && bootefi selftest
Testing EFI API implementation
Selected test: 'manage protocols'
Setting up 'manage protocols'
Setting up 'manage protocols' succeeded
Executing 'manage protocols'
Executing 'manage protocols' succeeded
Tearing down 'manage protocols'
Tearing down 'manage protocols' succeeded
Summary: 0 failures

=> bootefi selftest
Testing EFI API implementation
Selected test: 'manage protocols'
Setting up 'manage protocols'
lib/efi_selftest/efi_selftest_manageprotocols.c(88):
ERROR: InstallProtocolInterface failed
lib/efi_selftest/efi_selftest.c(89):
ERROR: Setting up 'manage protocols' failed
Tearing down 'manage protocols'
Tearing down 'manage protocols' succeeded
Summary: 1 failures

The reason is that we don't set the handles to NULL after deleting and
freeing them.  As a result the subsequent protocol installation will try
to use an existing handle which we just removed that from our object list.

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>