Fix clang ioc warning due to NULL src_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: I12dffcd5fb1002b2b2dabb083c8726650e4b5f08
Merge "Fix clang ioc warning due to NULL src_mi pointer."