Initial error handling code

src/core/SkXfermode.cpp

Index: src/core/SkXfermode.cpp
diff --git a/src/core/SkXfermode.cpp b/src/core/SkXfermode.cpp
index 8c6eb2ca687f254c57f8ef6e19a8f72405f30c9e..9daad7130cbe50e8fe054859ecee5fe5647e4fb5 100644
--- a/src/core/SkXfermode.cpp
+++ b/src/core/SkXfermode.cpp
@@ -12,6 +12,7 @@
 #include "SkFlattenableBuffers.h"
 #include "SkMathPriv.h"
 #include "SkString.h"
+#include "SkValidationUtils.h"
 
 SK_DEFINE_INST_COUNT(SkXfermode)
 
@@ -1402,13 +1403,20 @@ public:
 protected:
     SkProcCoeffXfermode(SkFlattenableReadBuffer& buffer) : INHERITED(buffer) {
         fMode = (SkXfermode::Mode)buffer.read32();
-
-        const ProcCoeff& rec = gProcCoeffs[fMode];
-        // these may be valid, or may be CANNOT_USE_COEFF
-        fSrcCoeff = rec.fSC;
-        fDstCoeff = rec.fDC;
-        // now update our function-ptr in the super class
-        this->INHERITED::setProc(rec.fProc);
+        bool modeIsValid = SkIsValidMode(fMode);
+        if (modeIsValid) {
+            const ProcCoeff& rec = gProcCoeffs[fMode];
+            // these may be valid, or may be CANNOT_USE_COEFF
+            fSrcCoeff = rec.fSC;
+            fDstCoeff = rec.fDC;
+            // now update our function-ptr in the super class
+            this->INHERITED::setProc(rec.fProc);
+        } else {
+            fSrcCoeff = fDstCoeff = CANNOT_USE_COEFF;
+        }
+        buffer.validate(modeIsValid &&
+                        (SkIsValidCoeff(fSrcCoeff) || (fSrcCoeff == CANNOT_USE_COEFF)) &&
+                        (SkIsValidCoeff(fDstCoeff) || (fDstCoeff == CANNOT_USE_COEFF)));
     }
 
     virtual void flatten(SkFlattenableWriteBuffer& buffer) const SK_OVERRIDE {