vp9_cx_iface,encoder_encode: check validate_img return

before calling vp9_apply_encoding_flags() which may crash if the
resolution was invalid. this is the same change as:
c0523090b vp8e_encode: check validate_config return

BUG=https://bugzilla.mozilla.org/show_bug.cgi?id=1315288

Change-Id: Icd2aab322422e83d3a778fca6d7789e5000239d7
1 file changed