Fill the initial JBIG2 buffer with 0xFF only if defaultPixelValue is set

Per the TypedArray specification, ArrayBuffers are initialized with zeros.

http://www.khronos.org/registry/typedarray/specs/latest/#5
Merge pull request #4346 from p01/patch-2

Fill the initial JBIG2 buffer with 0xFF only if defaultPixelValue is set