[OpenMW-CS] Load master index from refId to mContentFile

Fixes issue 5675. The idea is to save the first byte (0x00) to mContentFile, but also keep master index data in mIndex.

In theory, it should not be necessary to have the master index in both variables, but I don't think it's a big deal. If the first byte of refId that contains master index, is loaded to mContentFile, but then zeroed, openmw-cs loads the files properly, but saving fails. Zeroing permanently is unnecessary, zeroing is necessary temporarily to connect the refId to the correct instance. When the first byte is kept in mIndex (as in this MR), saving works properly.