av1_set_reference_dec: omit init of ref_buf

The initialization is not needed, because we immediately assign a value
to ref_buf.

Change-Id: I4e568339afce7959701b11a374a4c60d8c6ff36f

av1/decoder/decoder.c

diff --git a/av1/decoder/decoder.c b/av1/decoder/decoder.c
index 21db273..fac6007 100644
--- a/av1/decoder/decoder.c
+++ b/av1/decoder/decoder.c
@@ -287,11 +287,10 @@
                                       int use_external_ref,
                                       YV12_BUFFER_CONFIG *sd) {
   const int num_planes = av1_num_planes(cm);
-  YV12_BUFFER_CONFIG *ref_buf = NULL;
   // Ensure that aom_internal_error() calls longjmp().
   assert(cm->error->setjmp);
   // Get the destination reference buffer.
-  ref_buf = get_ref_frame(cm, idx);
+  YV12_BUFFER_CONFIG *ref_buf = get_ref_frame(cm, idx);
 
   if (ref_buf == NULL) {
     aom_internal_error(cm->error, AOM_CODEC_ERROR, "No reference frame");