[crypto] Allow for zero-length big integer literals

[crypto] Allow for zero-length big integer literals

Ensure that zero-length big integer literals are treated as containing
a zero value.  Avoid tests on every big integer arithmetic operation
by ensuring that bigint_required_size() always returns a non-zero
value: the zero-length tests can therefore be restricted to only
bigint_init() and bigint_done().

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