Verify number of ex flags matches number of ex items.
Verify number of ex flags matches number of ex items.

Currently the JBig2 decoder can leak subimages in the case where we mark
more items in EXFLAGS then we have SDNUMEXSYMS. This Cl checks for this
condition and fails the decode if it happens.

BUG=chromium:654365

Review-Url: https://codereview.chromium.org/2419553002

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