Fix SkTArray operator= to work with self assignment

BUG=skia:

Change-Id: I2a403a7ccbb87a030757f3e57d2ea53503f72512
Reviewed-on: https://skia-review.googlesource.com/10012
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
2 files changed