Do not create empty color table if it does not exist

BUG=skia:

Review URL: https://codereview.chromium.org/1019543003
diff --git a/src/codec/SkCodec_libbmp.cpp b/src/codec/SkCodec_libbmp.cpp
index 62cda95..e9551cb 100644
--- a/src/codec/SkCodec_libbmp.cpp
+++ b/src/codec/SkCodec_libbmp.cpp
@@ -570,7 +570,9 @@
     }
 
     // Set the color table and return true on success
-    fColorTable.reset(SkNEW_ARGS(SkColorTable, (colorTable, maxColors)));
+    if (maxColors > 0) {
+        fColorTable.reset(SkNEW_ARGS(SkColorTable, (colorTable, maxColors)));
+    }
     return true;
 }