Fix more null-checks in SVGLengthContext::convertValueFrom*

The following methods in SVGLengthContext:

 convertValueFromUserUnitsToCHS
 convertValueFromUserUnitsToEXS
 convertValueFromEXSToUserUnits

needs the same treatment as convertValueFromCHSToUserUnits got in
https://chromiumcodereview.appspot.com/2445463002.

R=pdr@chromium.org,eae@chromium.org
BUG=657438,658585,658613

Review-Url: https://codereview.chromium.org/2449433002
Cr-Commit-Position: refs/heads/master@{#427080}
1 file changed