vpx_codec_decode: check data size

When a valid data pointer is given make sure the size is greater than
zero.
A previous check for vp9 was incorrectly removed in:
70500747 Make the api behavior conform to api spec.

No semantics for valid pointers + 0-sized frames are defined for VPx
codecs, so move the check to vpx_codec_decode(). This avoids an assert
in vp9.

+ add some basic invalid param testing for decoder init/decode/destroy

Change-Id: I99f9cef6076d15874fd72ac973f2685d8a2353c3