libs/gst/base/gstbasesrc.c: When the subclass did not set caps on outgoing buffers, configure the caps we negotiated ...

Original commit message from CVS:
* libs/gst/base/gstbasesrc.c: (gst_base_src_get_range),
(gst_base_src_start):
When the subclass did not set caps on outgoing buffers, configure the
caps we negotiated on the source pad.
When the typefind helper does not find caps, error out properly instead
of doing things with NULL caps.