Fix libvpx VP9 decoder dr memory errors
Fix libvpx VP9 decoder dr memory errors

Fixed dr memory errors reported in Issue 736:
https://code.google.com/p/webm/issues/detail?id=736

All elements in left_col buffer need to be initialized to ensure
the correctness of SIMD operations in x86 optimized code.

Change-Id: I8e7f26ab45cca8099c1f9342bcf852f828bda7e4

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