Add mode validation to PtProcRec::init.

BUG=486946

Review URL: https://codereview.chromium.org/1142563003

src/core/SkDraw.cpp

@@ -443,6 +443,10 @@ static void aa_square_proc(const PtProcRec& rec, const SkPoint devPts[],
 // If this guy returns true, then chooseProc() must return a valid proc
 bool PtProcRec::init(SkCanvas::PointMode mode, const SkPaint& paint,
                      const SkMatrix* matrix, const SkRasterClip* rc) {
+    if ((unsigned)mode > (unsigned)SkCanvas::kPolygon_PointMode) {
+        return false;
+    }
+
     if (paint.getPathEffect()) {
         return false;
     }