Don't allow MemoryFiles of negative length.

Prevents us from converting a (signed) jint into an
(unsigned) size_t and having horrible things happen.

Change-Id: I0f04e2eb9852ae7fc49b435fd0974f56e86751a4
Signed-off-by: houst0nn <houstonn@gmail.com>
Merge "Don't allow MemoryFiles of negative length." into kitkat