Fix null crash in CheckTrailer.

Fix null crash in CheckTrailer.

BUG=450871

[Bo Xu, 2015-01-22 18:59:15]

bo_xu@foxitsoftware.com changed reviewers:
+ rickyz@chromium.org

[Bo Xu, 2015-01-22 18:59:15]

Ricky, please review this.

[Tom Sepez, 2015-01-22 19:06:32]

tsepez@chromium.org changed reviewers:
+ tsepez@chromium.org
- rickyz@chromium.org

[Tom Sepez, 2015-01-22 19:06:32]

LGTM (I reviewed the original patch and missed this entirely).

[Tom Sepez, 2015-01-22 19:07:30]

On 2015/01/22 19:06:32, Tom Sepez wrote:
> LGTM (I reviewed the original patch and missed this entirely).
We should be able to make a trivial test for this, right?  Adding a botched
trailer and loading it from embeddertests?

[Bo Xu, 2015-01-22 19:08:19]

On 2015/01/22 19:06:32, Tom Sepez wrote:
> LGTM (I reviewed the original patch and missed this entirely).

Accidentally uploaded that one with wrong base url.

[Bo Xu, 2015-01-22 19:09:47]

On 2015/01/22 19:07:30, Tom Sepez wrote:
> On 2015/01/22 19:06:32, Tom Sepez wrote:
> > LGTM (I reviewed the original patch and missed this entirely).
> We should be able to make a trivial test for this, right?  Adding a botched
> trailer and loading it from embeddertests?

Sure, shall I do in this patch or a separate one?

[Tom Sepez, 2015-01-22 19:11:06]

On 2015/01/22 19:09:47, Bo Xu wrote:
> On 2015/01/22 19:07:30, Tom Sepez wrote:
> > On 2015/01/22 19:06:32, Tom Sepez wrote:
> > > LGTM (I reviewed the original patch and missed this entirely).
> > We should be able to make a trivial test for this, right?  Adding a botched
> > trailer and loading it from embeddertests?
> 
> Sure, shall I do in this patch or a separate one?

It's best to land them together.  You can write the test, see that it crashes
against the old code, and then see that it works against the new code (testing
the test as it were).

[Tom Sepez, 2015-01-23 02:18:18]

On 2015/01/22 19:11:06, Tom Sepez wrote:
> On 2015/01/22 19:09:47, Bo Xu wrote:
> > On 2015/01/22 19:07:30, Tom Sepez wrote:
> > > On 2015/01/22 19:06:32, Tom Sepez wrote:
> > > > LGTM (I reviewed the original patch and missed this entirely).
> > > We should be able to make a trivial test for this, right?  Adding a
botched
> > > trailer and loading it from embeddertests?
> > 
> > Sure, shall I do in this patch or a separate one?
> 
I cobbled together a few tests at https://codereview.chromium.org/872563002/,
please take a look.

[Bo Xu, 2015-01-24 20:23:00]

Close in regard to land of https://codereview.chromium.org/872563002/