Fix crash in _CMapLookupCallback

fix a crash issue in _CMapLookupCallback

BUG=382242
R=palmer@chromium.org

Committed: https://pdfium.googlesource.com/pdfium/+/a26b328

[jun_fang, 2014-06-21 00:26:31]

Hi Chris,

I fixed this crash issue. It's because two objects have the same number which is
not compatible with the definitions in PDF reference. Before this fix, the first
object will be deleted and save the second one. However, the first object has
been referred somewhere. It will cause a crash when the first object is accessed
after released.So I changed the implementation to return when the given object
number can be found. 

Best regards,
Jun Fang

[palmer, 2014-06-25 23:21:44]

https://codereview.chromium.org/341333004/diff/1/core/src/fpdfapi/fpdf_parser...
File core/src/fpdfapi/fpdf_parser/fpdf_parser_objects.cpp (right):

https://codereview.chromium.org/341333004/diff/1/core/src/fpdfapi/fpdf_parser...
core/src/fpdfapi/fpdf_parser/fpdf_parser_objects.cpp:1333: return;
But the caller is going to expect that this function succeeded in updating
|pObj|, right? Is this fix going to cause other breakage (other crashes, PDF
semantics violations)?

[jun_fang, 2014-07-03 05:19:22]

An object with a new generation number shall be used to replace the object with
the old generation number. In other cases, don't need to handle but return.

[palmer, 2014-07-07 18:25:44]

> An object with a new generation number shall be used to replace the object
with
> the old generation number. In other cases, don't need to handle but return.

OK, LGTM. Thanks!

[jun_fang, 2014-07-07 21:07:21]

Committed patchset #2 manually as ra26b328 (presubmit successful).