tiffdec: fix EXTRASAMPLES check

the TIFFGetField() return for TIFFTAG_EXTRASAMPLES is defined as (count,
types array) [1]. previously the count was being checked rather than the
first element of the array to determine whether the alpha was associated
(pre-multiplied) and the result needed to be unmultiplied.

since:
9273e441 fix TIFF encoder regarding rgbA/RGBA

[1] http://www.libtiff.org/man/TIFFSetField.3t.html

Change-Id: I6e41be9d038fe8afb6d0aa3c8048925dc901113b

imageio/tiffdec.c

@@ -215,7 +215,8 @@ int ReadTIFF(const uint8_t* const data, size_t data_size,
       TIFFSwabArrayOfLong(raster, width * height);
 #endif
       // if we have an alpha channel, we must un-multiply from rgbA to RGBA
-      if (samples_per_px > 3 && extra_samples == 1) {
+      if (extra_samples == 1 && extra_samples_ptr != NULL &&
+          extra_samples_ptr[0] == EXTRASAMPLE_ASSOCALPHA) {
         uint32_t y;
         uint8_t* tmp = (uint8_t*)raster;
         for (y = 0; y < height; ++y) {