Fix clang ioc warning due to NULL mi pointer.

The warning only happens in VP9 encoder's first pass due to src_mi
is not set up yet. But it will not fail the encoder as left_mi and
above_mi are not used in the first_pass and they will be set up again
in the second pass.

Change-Id: I0713b4660d71e229e196654cb0970ba6b1574f28

vp9/common/vp9_onyxc_int.h

@@ -380,7 +380,8 @@ static INLINE void set_mi_row_col(MACROBLOCKD *xd, const TileInfo *const tile,
   xd->left_available  = (mi_col > tile->mi_col_start);
   if (xd->up_available) {
     xd->above_mi = xd->mi[-xd->mi_stride];
-    xd->above_mbmi = &xd->above_mi->mbmi;
+    // above_mi may be NULL in VP9 encoder's first pass.
+    xd->above_mbmi = xd->above_mi ? &xd->above_mi->mbmi : NULL;
   } else {
     xd->above_mi = NULL;
     xd->above_mbmi = NULL;
@@ -388,7 +389,8 @@ static INLINE void set_mi_row_col(MACROBLOCKD *xd, const TileInfo *const tile,
 
   if (xd->left_available) {
     xd->left_mi = xd->mi[-1];
-    xd->left_mbmi = &xd->left_mi->mbmi;
+    // left_mi may be NULL in VP9 encoder's first pass.
+    xd->left_mbmi = xd->left_mi ? &xd->left_mi->mbmi : NULL;
   } else {
     xd->left_mi = NULL;
     xd->left_mbmi = NULL;