Fix clang ioc warning due to NULL mi pointer.
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

running770/libvpx,jmvalin/aom,liqianggao/libvpx,zofuthan/libvpx,GrokImageCompression/aom,openpeer/libvpx_new,ShiftMediaProject/libvpx,smarter/aom,liqianggao/libvpx,felipebetancur/libvpx,mbebenita/aom,shacklettbp/aom,GrokImageCompression/aom,goodleixiao/vpx,mwgoldsmith/libvpx,Topopiccione/libvpx,shacklettbp/aom,smarter/aom,jmvalin/aom,shyamalschandra/libvpx,liqianggao/libvpx,liqianggao/libvpx,mwgoldsmith/vpx,stewnorriss/libvpx,ittiamvpx/libvpx-1,thdav/aom,luctrudeau/aom,stewnorriss/libvpx,zofuthan/libvpx,ShiftMediaProject/libvpx,shyamalschandra/libvpx,luctrudeau/aom,stewnorriss/libvpx,webmproject/libvpx,Topopiccione/libvpx,Topopiccione/libvpx,felipebetancur/libvpx,jmvalin/aom,openpeer/libvpx_new,mbebenita/aom,ittiamvpx/libvpx-1,shyamalschandra/libvpx,liqianggao/libvpx,thdav/aom,ShiftMediaProject/libvpx,GrokImageCompression/aom,zofuthan/libvpx,VTCSecureLLC/libvpx,stewnorriss/libvpx,shyamalschandra/libvpx,mwgoldsmith/libvpx,thdav/aom,mwgoldsmith/vpx,running770/libvpx,VTCSecureLLC/libvpx,smarter/aom,jmvalin/aom,VTCSecureLLC/libvpx,VTCSecureLLC/libvpx,ittiamvpx/libvpx-1,running770/libvpx,thdav/aom,mwgoldsmith/vpx,shacklettbp/aom,felipebetancur/libvpx,running770/libvpx,ittiamvpx/libvpx-1,smarter/aom,luctrudeau/aom,felipebetancur/libvpx,openpeer/libvpx_new,shacklettbp/aom,Topopiccione/libvpx,thdav/aom,mwgoldsmith/vpx,ShiftMediaProject/libvpx,thdav/aom,jmvalin/aom,shacklettbp/aom,mbebenita/aom,ittiamvpx/libvpx-1,mbebenita/aom,webmproject/libvpx,mwgoldsmith/vpx,shyamalschandra/libvpx,mwgoldsmith/vpx,GrokImageCompression/aom,stewnorriss/libvpx,shacklettbp/aom,luctrudeau/aom,goodleixiao/vpx,running770/libvpx,running770/libvpx,smarter/aom,webmproject/libvpx,jmvalin/aom,stewnorriss/libvpx,zofuthan/libvpx,liqianggao/libvpx,openpeer/libvpx_new,VTCSecureLLC/libvpx,VTCSecureLLC/libvpx,GrokImageCompression/aom,webmproject/libvpx,mbebenita/aom,Topopiccione/libvpx,webmproject/libvpx,shyamalschandra/libvpx,webmproject/libvpx,mbebenita/aom,openpeer/libvpx_new,smarter/aom,openpeer/libvpx_new,goodleixiao/vpx,goodleixiao/vpx,felipebetancur/libvpx,luctrudeau/aom,goodleixiao/vpx,luctrudeau/aom,ittiamvpx/libvpx-1,mwgoldsmith/libvpx,GrokImageCompression/aom,Topopiccione/libvpx,mbebenita/aom,mwgoldsmith/libvpx,felipebetancur/libvpx,goodleixiao/vpx,mwgoldsmith/libvpx,mbebenita/aom,mwgoldsmith/libvpx,zofuthan/libvpx,zofuthan/libvpx,mbebenita/aom,ShiftMediaProject/libvpx