[CFL] Check for uninitialized signs

To avoid writting out uninitialized signs for CfL alpha, checks are
added in the write_cfl_alphas function.

Change-Id: I832875e496a6318132c82bc1e5406dc95ae9c055

av1/encoder/bitstream.c

@@ -1703,6 +1703,12 @@ static void write_cfl_alphas(FRAME_CONTEXT *const frame_ctx, int skip, int ind,
     assert(signs[CFL_PRED_U] == CFL_SIGN_POS);
     assert(signs[CFL_PRED_V] == CFL_SIGN_POS);
   } else {
+    // Check for uninitialized signs
+    if (cfl_alpha_codes[ind][CFL_PRED_U] == 0)
+      assert(signs[CFL_PRED_U] == CFL_SIGN_POS);
+    if (cfl_alpha_codes[ind][CFL_PRED_V] == 0)
+      assert(signs[CFL_PRED_V] == CFL_SIGN_POS);
+
     // Write a symbol representing a combination of alpha Cb and alpha Cr.
     aom_write_symbol(w, ind, frame_ctx->cfl_alpha_cdf, CFL_ALPHABET_SIZE);