Cast allocated pointers to proper types.

Change-Id: I473135d28e1d7f16bd37652ffeaad62dfd9bbe73
diff --git a/video_reader.c b/video_reader.c
index 5cfe02f..6a96af9 100644
--- a/video_reader.c
+++ b/video_reader.c
@@ -40,7 +40,7 @@
 
   if (mem_get_le16(header + 4) != 0) return NULL;  // Wrong IVF version
 
-  reader = calloc(1, sizeof(*reader));
+  reader = (AvxVideoReader *)calloc(1, sizeof(*reader));
   if (!reader) return NULL;  // Can't allocate AvxVideoReader
 
   reader->file = file;