Fix a compile warning under certain configs

Change-Id: Ic35c643764bd50f9264525acb53a313ae23a7448

av1/encoder/firstpass.c

diff --git a/av1/encoder/firstpass.c b/av1/encoder/firstpass.c
index 47e562b..6a2fe22 100644
--- a/av1/encoder/firstpass.c
+++ b/av1/encoder/firstpass.c
@@ -975,7 +975,7 @@
       fp_block_size_height >> (this_frame->y_height > this_frame->uv_height);
 
   MV best_ref_mv = kZeroMv;
-  MV last_mv;
+  MV last_mv = { 0, 0 };
 
   // Reset above block coeffs.
   xd->up_available = (mb_row_in_tile != 0);