Fix regression in CD-ROM READ(6) command (#231)

Block count of 0 means 256 blocks in SCSI READ(6) command.
This bug was introduced in commit fcac5ae and is present
in versions 2023-05-24 and 2023-05-25.

src/BlueSCSI_cdrom.cpp

@@ -1742,6 +1742,7 @@ extern "C" int scsiCDRomCommand()
             (((uint32_t) scsiDev.cdb[2]) << 8) +
             scsiDev.cdb[3];
         uint32_t blocks = scsiDev.cdb[4];
+        if (blocks == 0) blocks = 256;
 
         doReadCD(lba, blocks, 0, 0x10, 0, true);
     }