process_pipeline: Set the thumbnail row buffer to correct size

Otherwise it's saving 2*image->width bytes of uninitialized memory
image->height times into the DNG file.

src/process_pipeline.c

@@ -401,7 +401,7 @@ process_image_for_capture(const uint8_t *image, int count)
 	// Write black thumbnail, only windows uses this
 	{
 		unsigned char *buf =
-			(unsigned char *)calloc(1, (int)mode.width >> 4);
+			(unsigned char *)calloc(1, (mode.width >> 4) * 3);
 		for (int row = 0; row < (mode.height >> 4); row++) {
 			TIFFWriteScanline(tif, buf, row, 0);
 		}