Cast (height - 1) to ptrdiff_t in aom_img_flip()

aom_img_flip() casts (height - 1) to `signed` before multiplying it with
stride. Change the `signed` casts to `ptrdiff_t` casts to avoid
overflows on 64-bit systems.

Since the img->planes[] buffers have been allocated successfully, the
(height - 1) * stride products can be represented in the `ptrdiff_t`
type, so the `ptrdiff_t` casts won't truncate (height - 1).

Change-Id: I3ab4b887e832eb2e663d58ccdf9ec98e8375d26c

aom/src/aom_image.c

diff --git a/aom/src/aom_image.c b/aom/src/aom_image.c
index 881429a..69b8f26 100644
--- a/aom/src/aom_image.c
+++ b/aom/src/aom_image.c
@@ -12,6 +12,7 @@
 #include <assert.h>
 #include <limits.h>
 #include <stdbool.h>
+#include <stddef.h>
 #include <stdlib.h>
 #include <string.h>
 
@@ -296,27 +297,21 @@
   const unsigned int chroma_height =
       (img->d_h + img->y_chroma_shift) >> img->y_chroma_shift;
 
-  /* Note: In the pointer adjustment calculations, we want the rhs to be
-   * promoted to a signed type. Section 6.3.1.8 of the ISO C99 standard
-   * indicates that if the first operand of the multiplication is unsigned, the
-   * stride will be promoted to unsigned, causing errors when the lhs is a
-   * larger type than the rhs.
-   */
   if (img->planes[AOM_PLANE_Y]) {
     img->planes[AOM_PLANE_Y] +=
-        (signed)(img->d_h - 1) * img->stride[AOM_PLANE_Y];
+        (ptrdiff_t)(img->d_h - 1) * img->stride[AOM_PLANE_Y];
   }
   img->stride[AOM_PLANE_Y] = -img->stride[AOM_PLANE_Y];
 
   if (img->planes[AOM_PLANE_U]) {
     img->planes[AOM_PLANE_U] +=
-        (signed)(chroma_height - 1) * img->stride[AOM_PLANE_U];
+        (ptrdiff_t)(chroma_height - 1) * img->stride[AOM_PLANE_U];
   }
   img->stride[AOM_PLANE_U] = -img->stride[AOM_PLANE_U];
 
   if (img->planes[AOM_PLANE_V]) {
     img->planes[AOM_PLANE_V] +=
-        (signed)(chroma_height - 1) * img->stride[AOM_PLANE_V];
+        (ptrdiff_t)(chroma_height - 1) * img->stride[AOM_PLANE_V];
   }
   img->stride[AOM_PLANE_V] = -img->stride[AOM_PLANE_V];
 }