Silence another PNG print statement

It can still be turned on by using setting
c_suppressPNGImageDecoderWarnings to true.

Review URL: https://codereview.chromium.org/1512293003
diff --git a/src/images/SkImageDecoder_libpng.cpp b/src/images/SkImageDecoder_libpng.cpp
index 2dead58..6acbf29 100644
--- a/src/images/SkImageDecoder_libpng.cpp
+++ b/src/images/SkImageDecoder_libpng.cpp
@@ -128,7 +128,9 @@
 #endif
 
 static void sk_error_fn(png_structp png_ptr, png_const_charp msg) {
-    SkDEBUGF(("------ png error %s\n", msg));
+    if (!c_suppressPNGImageDecoderWarnings) {
+        SkDEBUGF(("------ png error %s\n", msg));
+    }
     longjmp(png_jmpbuf(png_ptr), 1);
 }