Bugfix for frame type with kf min/max dist of 0

All frames should be keyframes when "--kf-min-dist=0 --kf-max-dist=0" is
given.

But earlier, only the 1st frame was keyframe in this case.

BUG=aomedia:2725

Change-Id: I376eda9be242cc8178a59f023bd00a2219c5a74b
(cherry picked from commit a4bed750abd6904d54f416b22d2c1aa5337332f6)

av1/encoder/pass2_strategy.c

diff --git a/av1/encoder/pass2_strategy.c b/av1/encoder/pass2_strategy.c
index 6adc1fb..dae4847 100644
--- a/av1/encoder/pass2_strategy.c
+++ b/av1/encoder/pass2_strategy.c
@@ -2549,7 +2549,8 @@
   }
 
   // Keyframe and section processing.
-  if (rc->frames_to_key == 0 || (frame_flags & FRAMEFLAGS_KEY)) {
+  if (rc->frames_to_key <= 0 || (frame_flags & FRAMEFLAGS_KEY)) {
+    assert(rc->frames_to_key >= -1);
     FIRSTPASS_STATS this_frame_copy;
     this_frame_copy = this_frame;
     frame_params->frame_type = KEY_FRAME;