efi/libstub: Free correct pointer on failure

commit 06d39d79cbd5a91a33707951ebf2512d0e759847 upstream.

cmdline_ptr is an out parameter, which is not allocated by the function
itself, and likely points into the caller's stack.

cmdline refers to the pool allocation that should be freed when cleaning
up after a failure, so pass this instead to free_pool().

Fixes: 42c8ea3d ("efi: libstub: Factor out EFI stub entrypoint ...")
Cc: <stable@vger.kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>