[base16] Ensure base16_encode() always terminates its result string

base16_encode() will fail to generate a terminating NUL if the length
of the raw data is zero, since the loop calling sprintf() will never
execute.

Fix by explicitly terminating the result with a NUL.

Reported-by: Marin Hannache <git@mareo.fr>
Debugged-by: Marin Hannache <git@mareo.fr>
Tested-by: Marin Hannache <git@mareo.fr>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/base16.c

 	char *encoded_bytes = encoded;
 	size_t remaining = len;
 
+	/* Encode each byte */
 	for ( ; remaining-- ; encoded_bytes += 2 ) {
 		sprintf ( encoded_bytes, "%02x", *(raw_bytes++) );
 	}
 
+	/* Ensure terminating NUL exists even if length was zero */
+	*encoded_bytes = '\0';
+
 	DBG ( "Base16-encoded to \"%s\":\n", encoded );
 	DBG_HDA ( 0, raw, len );
 	assert ( strlen ( encoded ) == base16_encoded_len ( len ) );