Fix warning in FileStream.cpp.

StormLib/src/FileStream.cpp:1374:20: warning: converting ‘false’ to pointer type ‘TFileStream*’ [-Wconversion-null]
             return false;
                    ^
StormLib/src/FileStream.cpp:2266:24: warning: converting ‘false’ to pointer type ‘TFileStream*’ [-Wconversion-null]
                 return false;
                        ^