XOREOSTEX2TGA: Failure to convert many TPC files

[DrMcCoy]

(This was introduced with @Nostritius's ecfb314)

xoreostex2tga now fails to convert many KotOR TPCs, with a "Invalid UTF-8" error thrown. C_Wraid02.tpc is one example.

That example file has no attached TXI data, and I guess it tries to read other binary data as TXI, thus throwing. I.e. the offset after reading the mipmap data, before reading the TXI data, is not at the end of the file.

[DrMcCoy]

Fixed with #44 :)