gst/gstcaps.c: Fix crasher when passed NULL. Doc clarification.

gst/gstcaps.c: Fix crasher when passed NULL. Doc clarification.

Original commit message from CVS:
* gst/gstcaps.c: (gst_caps_replace):
Fix crasher when passed NULL. Doc clarification.
Optimize for the trivial case.
* gst/gstpipeline.c: (gst_pipeline_change_state):
Small cleanups.
* libs/gst/base/gstbasesrc.c: (gst_base_src_loop):
Small documentation cleanup.
* libs/gst/base/gstbasetransform.c:
(gst_base_transform_buffer_alloc):
Don't use silly gst_pad_get_negotiated_caps, GST_PAD_CAPS
is what we need and it avoids a whole lot of redundant
refcount operations.