libtpm: Compile with -Wextra

-Wextra enables a bunch of rather useful checks which this fixes.

Note this adds MIN() in tpm_gpt_set_lba1() so it may potentially fail
which is unlikely as the length comes from disk-label's block-size which
is used in other places.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>