Add a fix for a spurious assert in SkMatrixConvolutionImageFilter.

Add a fix for a spurious assert in SkMatrixConvolutionImageFilter.

When matrix convolution processes border pixels with zero width, it
asserts in getAddr32() with an invalid x coordinate. The assert is
harmless, since the returned pointer is never accessed (the next line
is a loop from left to right, which does nothing, since left == right).
However, the fix is simple: early out on an empty rect before entering
the outer loop.

R=sugoi@chromium.org

Committed: https://code.google.com/p/skia/source/detail?r=14497

[Stephen White, 2014-05-01 01:52:45]

sugoi@: PTAL. Thanks!

[Stephen White, 2014-05-01 02:15:00]

The CQ bit was checked by senorblanco@chromium.org

[commit-bot: I haz the power, 2014-05-01 02:15:16]

CQ is trying da patch. Follow status at
https://skia-tree-status.appspot.com/cq/senorblanco@chromium.org/265693005/1

[commit-bot: I haz the power, 2014-05-01 02:15:17]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-05-01 02:15:17]

No LGTM from a valid reviewer yet. Only full committers are accepted.
Even if an LGTM may have been provided, it was from a non-committer or
a provisional committer, _not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[sugoi, 2014-05-01 12:20:06]

On 2014/05/01 02:15:17, I haz the power (commit-bot) wrote:
> No LGTM from a valid reviewer yet. Only full committers are accepted.
> Even if an LGTM may have been provided, it was from a non-committer or
> a provisional committer, _not_ a full super star committer.
> See http://www.chromium.org/getting-involved/become-a-committer
> Note that this has nothing to do with OWNERS files.

lgtm

[Stephen White, 2014-05-01 12:45:27]

The CQ bit was checked by senorblanco@chromium.org

[commit-bot: I haz the power, 2014-05-01 12:45:31]

CQ is trying da patch. Follow status at
https://skia-tree-status.appspot.com/cq/senorblanco@chromium.org/265693005/1

[commit-bot: I haz the power, 2014-05-01 12:45:32]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-05-01 12:45:33]

No LGTM from a valid reviewer yet. Only full committers are accepted.
Even if an LGTM may have been provided, it was from a non-committer or
a provisional committer, _not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[Stephen White, 2014-05-01 14:03:44]

Committed patchset #1 manually as r14497 (presubmit successful).