[skpdf] Don't crash with degenerate stops in SkPDFGradientShader::Make.

This should still get the correct rendering, see the comment.

Bug: 535107486
Change-Id: I9d2940b8ca52589c2611960522641b3c4b4833c0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1294656
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Alexis Cruz-Ayala <alexisdavidc@google.com>
Auto-Submit: Emilio Cobos Álvarez <emilio@chromium.org>
Reviewed-by: Alexis Cruz-Ayala <alexisdavidc@google.com>