The ESRT hook call that I just added invokes the protocol before
retrieving a pointer to it, which interestingly enough did not
result in any crashes, nor did it get picked up by GCC. Clang did
notice, though, so let's fix it right away.
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>