restore intersect methods to use old-style empty-checks on their inputs

Bug introduced when we made isEmpty check for int32_t width/height

Bug:800804
Change-Id: I59799c88fb02f176c1545dd0edae050b510df079
Reviewed-on: https://skia-review.googlesource.com/93302
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
4 files changed