vpx_memset16: drop unnecessary local

+ add a cast

Change-Id: I2965e7867223aa25bf688c988629ac57b4971905

vpx_mem/vpx_mem.c

@@ -94,10 +94,9 @@ void vpx_free(void *memblk) {
 #if CONFIG_VP9_HIGHBITDEPTH
 void *vpx_memset16(void *dest, int val, size_t length) {
   size_t i;
-  void *orig = dest;
-  uint16_t *dest16 = dest;
+  uint16_t *dest16 = (uint16_t *)dest;
   for (i = 0; i < length; i++)
     *dest16++ = val;
-  return orig;
+  return dest;
 }
 #endif  // CONFIG_VP9_HIGHBITDEPTH