Fixes warnings:
```
../Source/Core/DiscIO/VolumeGC.cpp: In member function 'virtual u8 DiscIO::CVolumeGC::GetDiscNumber() const':
../Source/Core/DiscIO/VolumeGC.cpp:178:10: error: 'disc_number' may be used uninitialized in this function [-Werror=maybe-uninitialized]
return disc_number;
^
../Source/Core/DiscIO/VolumeWiiCrypted.cpp: In member function 'virtual u8 DiscIO::CVolumeWiiCrypted::GetDiscNumber() const':
../Source/Core/DiscIO/VolumeWiiCrypted.cpp:258:10: error: 'disc_number' may be used uninitialized in this function [-Werror=maybe-uninitialized]
return disc_number;
^
../Source/Core/DiscIO/VolumeWiiCrypted.cpp: In member function 'virtual IOS::ES::TMDReader DiscIO::CVolumeWiiCrypted::GetTMD() const':
../Source/Core/DiscIO/VolumeWiiCrypted.cpp:123:20: error: 'tmd_address' may be used uninitialized in this function [-Werror=maybe-uninitialized]
tmd_address <<= 2;
^
```
4cd9a2ef1e