fix misc asserts and checks found by fuzzer

src/effects/SkDashPathEffect.cpp

Index: src/effects/SkDashPathEffect.cpp
diff --git a/src/effects/SkDashPathEffect.cpp b/src/effects/SkDashPathEffect.cpp
index 6e10e5466d6220599adcb35c874c66e94ddc9318..ced0aab69a9930efbbd216127f776dd5d8c2770d 100644
--- a/src/effects/SkDashPathEffect.cpp
+++ b/src/effects/SkDashPathEffect.cpp
@@ -384,3 +384,17 @@ void SkDashPathEffect::toString(SkString* str) const {
     str->appendf("))");
 }
 #endif
+
+//////////////////////////////////////////////////////////////////////////////////////////////////
+
+SkPathEffect* SkDashPathEffect::Create(const SkScalar intervals[], int count, SkScalar phase) {
+    if ((count < 2) || !SkIsAlign2(count)) {
+        return nullptr;
+    }
+    for (int i = 0; i < count; i++) {
+        if (intervals[i] < 0) {
+            return nullptr;
+        }
+    }
+    return new SkDashPathEffect(intervals, count, phase);
+}