libs/gst/base/gstbasesrc.c: Use the result from gst_pad_set_caps() instead of assuming the element always accepted th...

Original commit message from CVS:
* libs/gst/base/gstbasesrc.c: (gst_base_src_default_negotiate):
Use the result from gst_pad_set_caps() instead of assuming the element
always accepted the caps computed by the default negotiate function.