Fix a sentinel length value used in CFX_CMapByteStringToPtr::SetAt
Fix a sentinel length value used in CFX_CMapByteStringToPtr::SetAt

It looks like an key length of 0 was incorrectly assumed to
denote an free entry.

Fixes a memory leak in FPDFViewEmbeddertest.Crasher_451830 due to values with
empty keys being incorrectly overwritten.

R=tsepez@chromium.org, thestig@chromium.org

Review URL: https://codereview.chromium.org/1534953002 .

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