[efi] Nullify interfaces and leak memory on uninstallation failure

[efi] Nullify interfaces and leak memory on uninstallation failure

The UEFI specification allows uninstallation of a protocol interface
to fail.  There is no sensible way for code to react to this, since
uninstallation is likely to be taking place on a code path that cannot
itself fail (e.g. a code path that is itself a failure path).

Where the protocol structure exists within a dynamically allocated
block of memory, this leads to possible use-after-free bugs.  Work
around this unfortunate design choice by nullifying the protocol
(i.e. overwriting the method pointers with no-ops) and leaking the
memory containing the protocol structure.

Signed-off-by: Michael Brown <mcb30@ipxe.org>