Initialize sf_identity during compressor init

Backported from libavm where there are some crashes in
certain configurations due to uninitialized sf_identity.

Change-Id: I33c6a39274b0ec6d4a50997174864a03177a3052
diff --git a/av1/encoder/encoder.c b/av1/encoder/encoder.c
index e9a744f..0d5b695 100644
--- a/av1/encoder/encoder.c
+++ b/av1/encoder/encoder.c
@@ -648,6 +648,9 @@
   resize_pending_params->width = 0;
   resize_pending_params->height = 0;
 
+  // Setup identity scale factor
+  av1_setup_scale_factors_for_frame(&cm->sf_identity, 1, 1, 1, 1);
+
   init_buffer_indices(&cpi->force_intpel_info, cm->remapped_ref_idx);
 
   av1_noise_estimate_init(&cpi->noise_estimate, cm->width, cm->height);