[base64] Avoid overrunning input data buffer

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

src/core/base64.c

 	uint8_t *encoded_bytes = ( ( uint8_t * ) encoded );
 	size_t raw_bit_len = ( 8 * len );
 	unsigned int bit;
+	unsigned int byte;
+	unsigned int shift;
 	unsigned int tmp;
 
 	for ( bit = 0 ; bit < raw_bit_len ; bit += 6 ) {
-		tmp = ( ( raw_bytes[ bit / 8 ] << ( bit % 8 ) ) |
-			( raw_bytes[ bit / 8 + 1 ] >> ( 8 - ( bit % 8 ) ) ) );
+		byte = ( bit / 8 );
+		shift = ( bit % 8 );
+		tmp = ( raw_bytes[byte] << shift );
+		if ( ( byte + 1 ) < len )
+			tmp |= ( raw_bytes[ byte + 1 ] >> ( 8 - shift ) );
 		tmp = ( ( tmp >> 2 ) & 0x3f );
 		*(encoded_bytes++) = base64[tmp];
 	}