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.h

@@ -59,6 +59,11 @@ int ExUtilWriteFile(const char* const file_name,
 void ExUtilCopyPlane(const uint8_t* src, int src_stride,
                      uint8_t* dst, int dst_stride, int width, int height);
 
+//------------------------------------------------------------------------------
+
+// Returns 0 in case of overflow of nmemb * size.
+int ImgIoUtilCheckSizeArgumentsOverflow(uint64_t nmemb, size_t size);
+
 //------------------------------------------------------------------------------
 // WebP decoding