videodecoder: decide_allocation w/ caps = NULL

Submitted by Ilya Konstantinov

Link to original bug (#748521)

Description

In gst_video_decoder_decide_allocation_default:

gst_query_parse_allocation (query, &outcaps, NULL);
gst_video_info_init (&vinfo);
if (outcaps)
gst_video_info_from_caps (&vinfo, outcaps);

However, outcaps == NULL is not legit to begin with, and will cause an assertion later in:

gst_buffer_pool_config_validate_params (config, outcaps, size, min,
max)

I think the 'if (outcaps)' check can be discarded.

Additionally, we might want to fail fast if !outcaps -- both in gst_video_decoder_decide_allocation_default and gst_base_src_decide_allocation_default.