film-grain: Fix some ASAN/segfault bugs
film-grain: Fix some ASAN/segfault bugs

* dealloc_arrays() calculated num_pos_chroma differently to
  init_arrays(). This caused us to read past the allocated length
  of *pred_pos_chroma in the case where params->num_y_points == 0.

* In the highbitdepth path of extend_even(), we were misusing
  the 'stride' attribute from an aom_image_t. This stride is given
  in bytes, but we were using it as a stride in 'uint16_t's.
  This sometimes lead to segfaults when applying film grain
  to high bit depth videos.

BUG=aomedia:1729

Change-Id: Ibf8825d2679265149377a14c7c0aec75c01d221c

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom