KEYS: Check hex2bin()'s return when generating an asymmetric key ID

As it stands, the code to generate an asymmetric key ID prechecks the hex
string it is given whilst determining the length, before it allocates the
buffer for hex2bin() to translate into - which mean that checking the result of
hex2bin() is redundant.

Unfortunately, hex2bin() is marked as __must_check, which means that the
following warning may be generated if the return value isn't checked:

	crypto/asymmetric_keys/asymmetric_type.c: In function
	asymmetric_key_hex_to_key_id:
	crypto/asymmetric_keys/asymmetric_type.c:110: warning: ignoring return
	value of hex2bin, declared with attribute warn_unused_result

The warning can't be avoided by casting the result to void.

Instead, use strlen() to check the length of the string and ignore the fact
that the string might not be entirely valid hex until after the allocation has
been done - in which case we can use the result of hex2bin() for this.

Signed-off-by: David Howells <dhowells@redhat.com>