Fix read size 0 in shared ppb_file_io.
Fix read size 0 in shared ppb_file_io.

We check that the src and dst pointers are non-null, however a read of size
zero is a NOP.  Typically library behavior is to ignore invalid arguments when
no work is actually done.

BUG=110087
R=brettw@chromium.org

Review URL: http://codereview.chromium.org/9695021

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@126461 0039d316-1c4b-4281-b951-d872f2087c98

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