Allow nullptr in Style[Non]InheritedVariables::SetVariable.

This does not currently cause a crash, entirely because of luck. A value
of nullptr is passed to SetVariable whenever an invalid var()-reference
is used (for example), but it just so happens that needs_resolution_ is
always true in this case, so it short circuits before attempting to de-
reference 'value'.

R=futhark@chromium.org

Bug: 641877
Change-Id: I8b3776a58efb82b5deef74b50561d2ebfd57b817
Reviewed-on: https://chromium-review.googlesource.com/1235725
Commit-Queue: Anders Ruud <andruud@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593136}
3 files changed