fix misc asserts and checks found by fuzzer

src/effects/SkAlphaThresholdFilter.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkAlphaThresholdFilter.h"
 #include "SkBitmap.h"
 #include "SkDevice.h"
@@ skipping 27 matching lines @@
     SkRegion fRegion;
     SkScalar fInnerThreshold;
     SkScalar fOuterThreshold;
     typedef SkImageFilter INHERITED;
 };
 
 SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_START(SkAlphaThresholdFilter)
     SK_DEFINE_FLATTENABLE_REGISTRAR_ENTRY(SkAlphaThresholdFilterImpl)
 SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_END
 
+static bool outside_unit(SkScalar x) {
+    return x < 0 || x > 1;
+}
 
 SkImageFilter* SkAlphaThresholdFilter::Create(const SkRegion& region,
                                               SkScalar innerThreshold,
                                               SkScalar outerThreshold,
                                               SkImageFilter* input) {
+    if (outside_unit(innerThreshold) || outside_unit(outerThreshold) ||
+        innerThreshold > outerThreshold)
+    {
+        return nullptr;
+    }
     return new SkAlphaThresholdFilterImpl(region, innerThreshold, outerThreshold, input);
 }
 
 #if SK_SUPPORT_GPU
 #include "GrContext.h"
 #include "GrCoordTransform.h"
 #include "GrFragmentProcessor.h"
 #include "GrInvariantOutput.h"
 #include "GrTextureAccess.h"
 #include "effects/GrPorterDuffXferProcessor.h"
@@ skipping 264 matching lines @@
 void SkAlphaThresholdFilterImpl::flatten(SkWriteBuffer& buffer) const {
     this->INHERITED::flatten(buffer);
     buffer.writeScalar(fInnerThreshold);
     buffer.writeScalar(fOuterThreshold);
     buffer.writeRegion(fRegion);
 }
 
 bool SkAlphaThresholdFilterImpl::onFilterImageDeprecated(Proxy* proxy, const SkBitmap& src,
                                                          const Context& ctx, SkBitmap* dst,
                                                          SkIPoint* offset) const {
-    SkASSERT(src.colorType() == kN32_SkColorType);
 
     if (src.colorType() != kN32_SkColorType) {
         return false;
     }
 
     SkMatrix localInverse;
     if (!ctx.ctm().invert(&localInverse)) {
         return false;
     }
 
@@ skipping 49 matching lines @@
 }
 
 #ifndef SK_IGNORE_TO_STRING
 void SkAlphaThresholdFilterImpl::toString(SkString* str) const {
     str->appendf("SkAlphaThresholdImageFilter: (");
     str->appendf("inner: %f outer: %f", fInnerThreshold, fOuterThreshold);
     str->append(")");
 }
 #endif