imageio_util: add ImgIoUtilCheckSizeArgumentsOverflow

and use it to validate decoder allocations. fixes a crash in jpegdec at
least.

BUG=webp:312

Change-Id: Ia940590098f29510add6aad10a8dfe9e9ea46bf4
(cherry picked from commit bc86b7a8a1)

examples/example_util.c

@@ -279,3 +279,10 @@ void ExUtilCopyPlane(const uint8_t* src, int src_stride,
 }
 
 // -----------------------------------------------------------------------------
+
+int ImgIoUtilCheckSizeArgumentsOverflow(uint64_t nmemb, size_t size) {
+  const uint64_t total_size = nmemb * size;
+  return (total_size == (size_t)total_size);
+}
+
+// -----------------------------------------------------------------------------