WebPDecodeYUV: check u/v/stride/uv_stride ptrs

The buffers are made mandatory to match WebPDecodeYUVInto(), though this
conflicts with WebPIDecGetYUVA().

spotted by Oliver Kunz (okunz at google dot com)

Change-Id: Ic4740c53b75da6b93d4f3462303fb9be0ebfbd48

src/dec/webp_dec.c

@@ -658,6 +658,12 @@ uint8_t* WebPDecodeBGRA(const uint8_t* data, size_t data_size,
 uint8_t* WebPDecodeYUV(const uint8_t* data, size_t data_size,
                        int* width, int* height, uint8_t** u, uint8_t** v,
                        int* stride, int* uv_stride) {
+  // data, width and height are checked by Decode().
+  if (u == NULL || v == NULL || stride == NULL || uv_stride == NULL) {
+    return NULL;
+  }
+
+  {
     WebPDecBuffer output;   // only to preserve the side-infos
     uint8_t* const out = Decode(MODE_YUV, data, data_size,
                                 width, height, &output);
@@ -672,6 +678,7 @@ uint8_t* WebPDecodeYUV(const uint8_t* data, size_t data_size,
     }
     return out;
   }
+}
 
 static void DefaultFeatures(WebPBitstreamFeatures* const features) {
   assert(features != NULL);

src/webp/decode.h

@@ -84,7 +84,8 @@ WEBP_EXTERN uint8_t* WebPDecodeBGR(const uint8_t* data, size_t data_size,
 // planes are both (*width + 1) / 2 and (*height + 1) / 2.
 // Upon return, the Y buffer has a stride returned as '*stride', while U and V
 // have a common stride returned as '*uv_stride'.
-// Return NULL in case of error.
+// 'width' and 'height' may be NULL, the other pointers must not be.
+// Returns NULL in case of error.
 // (*) Also named Y'CbCr. See: https://en.wikipedia.org/wiki/YCbCr
 WEBP_EXTERN uint8_t* WebPDecodeYUV(const uint8_t* data, size_t data_size,
                                    int* width, int* height,