Fix SkRRect validation for negative types

SkRRect::fType is an int, so needs to be checked to see if it's
negative.

Bug: chromium: 768149
Change-Id: I6a9a019c736806d35dc1bd627fb4ddc735a0ac86
Reviewed-on: https://skia-review.googlesource.com/54024
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Reed <reed@google.com>

src/core/SkRRect.cpp

@@ -553,7 +553,7 @@ bool SkRRect::isValid() const {
     }
     bool patchesOfNine = radii_are_nine_patch(fRadii);
 
-    if (fType > kLastType) {
+    if (fType < 0 || fType > kLastType) {
         return false;
     }