VectorComparer should check null.

We use VectorComparer to compare WTF::Vector<T>.
If T is not primitive type, we use std::equal to compare.
By the way, we use the WTF::VectorBufferBase class for a base class of Vector.  VectorBufferBase doesn't alloc a array if we init it without a size argument.
As a conclusion, we pass NULL object to std::equal when we compare Vectors with the default constructor.
It is asserted especially on a Windows debug build.

BUG=

Review URL: https://codereview.chromium.org/332113002

git-svn-id: svn://svn.chromium.org/blink/trunk@176189 bbb929c8-8fbe-4397-9dbb-9b2b20218538