vp8: Validate references in vp8e_set/get_reference

Check that the Y, U, and V plane pointers are non-NULL in
vp8e_set_reference() and vp8e_get_reference() before
converting to YV12_BUFFER_CONFIG.

Added unittest.

Note that the PoC in the bug report has an error in the fuzz
driver: it calls the internal vpx_codec_control_() (bypassing
the type-checked vpx_codec_control() macro) and
passes an int instead of a vpx_ref_frame_t * pointer for
VP8_SET_REFERENCE.

Bug: 564326909
Change-Id: I2580ca9a53ab67bf06d79cbea0711300d1cf605a