Make SkString::appendf safe for long strings.

Bug: skia:7841
Change-Id: I5bc77f5230b63da74e42d756ab4a1fefcfab9926
Reviewed-on: https://skia-review.googlesource.com/123634
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
2 files changed