CIfo: more safe file opening

Check all functions' returns and reject certainly invalid IFO
files now. We could easily crash here before.
CIfo: more safe file saving
CMpcAudioRenderer: fix a warning
CBaseMuxerOutputPin: fix a warning

The casted value is basically si->dwOffset, which is DWORD
CWebClientSocket: fix a warning
CRenderersData: fix possible integer overflow
Merge pull request #86 from alexmarsev/warnings

Fix warnings