BaseSplitterFileEx: Fix an incorrect check.

This check was always false.
BaseSplitterFileEx: Simplify a check.

h.bitpersample is unsigned, checking if it's greater than 0 will always be true.
BufferFilter: Check for null pointers before actually using them.
ComPropertySheet: Remove a redundant check.
FGManagerBDA: Check for null pointer before actually using it.
IPinHook: Fix wrong sizeof.
PPageFileInfoClip: Use CComBSTR Length method when possible.