Fix assertion in basetransform when the subclass chooses not to allocate a buffer...

Fix assertion in basetransform when the subclass chooses not to allocate a buffer in prepare_buffer(), and make capsf...

Original commit message from CVS:
* libs/gst/base/gstbasetransform.c:
* plugins/elements/gstcapsfilter.c:
* tests/check/Makefile.am:
* tests/check/elements/.cvsignore:
* tests/check/elements/capsfilter.c:
Fix assertion in basetransform when the subclass chooses not to
allocate a buffer in prepare_buffer(), and make capsfilter error out
cleanly if requested to apply caps that don't completely specify the
buffer. Fixes #551509