Guard some uint32_t -> int cases in SkPictureData reading

Change-Id: I67e6a67a57bf83922d159083e359df1a8ce053c1
Reviewed-on: https://skia-review.googlesource.com/6275
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
1 file changed