Rather use default constructor of std::vector in LibraryAnimations::L…

[theHamsta]

…ibraryAnimations than mOpenAnimation(NULL)

This avoid the following warning:

../COLLADAStreamWriter/src/COLLADASWLibraryAnimations.cpp:65:97: warning: implicit conversion of NULL constant to 'std::vector::size_type' (aka 'unsigned long') [-Wnull-conversion]
            : Library ( streamWriter, CSWC::CSW_ELEMENT_LIBRARY_ANIMATIONS ), mOpenAnimations ( NULL )
                                                                              ~~~~~~~~~~~~~~~   ^~~~
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