Ignore PLTE chunks in non-palette PNGs

Bug: oss-fuzz:10020
Change-Id: I2dd0dd07983fcc851ea6ec0980a5a30dbe312b62
Reviewed-on: https://skia-review.googlesource.com/149261
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
diff --git a/third_party/libpng/pnglibconf.h b/third_party/libpng/pnglibconf.h
index aa45c5d..7fe747d 100644
--- a/third_party/libpng/pnglibconf.h
+++ b/third_party/libpng/pnglibconf.h
@@ -6,3 +6,4 @@
  */
 
 #include "../externals/libpng/scripts/pnglibconf.h.prebuilt"
+#undef PNG_READ_OPT_PLTE_SUPPORTED