lockPixels before calling getColorTable.

Fixes a bug where we try to dereference a NULL pointer.

Update the comment for SkBitmap::getColorTable, telling
the client that pixels must be locked for it to return
non-NULL.

R=edisonn@google.com

Review URL: https://codereview.chromium.org/15002002

git-svn-id: http://skia.googlecode.com/svn/trunk@9018 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/pdf/SkPDFImage.cpp b/src/pdf/SkPDFImage.cpp
index 04307be..102bd58 100644
--- a/src/pdf/SkPDFImage.cpp
+++ b/src/pdf/SkPDFImage.cpp
@@ -321,6 +321,7 @@
         insertName("ColorSpace", "DeviceGray");
     } else if (config == SkBitmap::kIndex8_Config ||
         config == SkBitmap::kRLE_Index8_Config) {
+        SkAutoLockPixels alp(bitmap);
         insert("ColorSpace",
                makeIndexedColorSpace(bitmap.getColorTable()))->unref();
     } else {