check (runtime) for null-pixels even when lock succeeds

src/core/SkMipMap.cpp

Index: src/core/SkMipMap.cpp
diff --git a/src/core/SkMipMap.cpp b/src/core/SkMipMap.cpp
index 77e0abc051157c0a438c138965edcf070f0791ab..032a60d0f1f6b420cc674b389f0ae7db8f178cb7 100644
--- a/src/core/SkMipMap.cpp
+++ b/src/core/SkMipMap.cpp
@@ -239,6 +239,10 @@ SkMipMap* SkMipMap::Build(const SkBitmap& src, SkDiscardableFactoryProc fact) {
         return NULL;
     }
     const SkPixmap& srcPixmap = srcUnlocker.pixmap();
+    // Try to catch where we might have returned NULL for src crbug.com/492818
+    if (NULL == srcPixmap.addr()) {
+        sk_throw();
+    }
 
     SkMipMap* mipmap;
     if (fact) {