Fix in calculation of key frame boost

The FIRSTPASS_STATS pointer passed to the function
calc_frame_boost was incorrect. This CL fixes the issue.

Change-Id: I51de19937e20a24dd1a22106c506a0308ca41cf1
diff --git a/av1/encoder/pass2_strategy.c b/av1/encoder/pass2_strategy.c
index bd3a29a..b3ffeb7 100644
--- a/av1/encoder/pass2_strategy.c
+++ b/av1/encoder/pass2_strategy.c
@@ -1592,7 +1592,7 @@
     if ((i <= rc->max_gf_interval) ||
         ((i <= (rc->max_gf_interval * 4)) && (decay_accumulator > 0.5))) {
       const double frame_boost =
-          calc_frame_boost(cpi, this_frame, 0, kf_max_boost);
+          calc_frame_boost(cpi, &next_frame, 0, kf_max_boost);
 
       // How fast is prediction quality decaying.
       if (!detect_flash(twopass, 0)) {