Merge to XFA: Cleanup JBig2_GeneralDecoder.cpp.

- FX_Alloc() can't fail and return.
- Use unique_ptr / remove gotos.
- NULL -> nullptr.
- Combine common code.

TBR=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1337823003 .

(cherry picked from commit d15cda90f598ce52fa601552577ef7fd8a89fce3)

Review URL: https://codereview.chromium.org/1349463002 .
2 files changed