Fix a heap overflow in CJBig2_Context::parseSymbolDict

BUG=476107
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1080893002

(cherry picked from commit eddab4425614e49146f904f00da4a664ba4b581b)

Review URL: https://codereview.chromium.org/1135133002
3 files changed