rpeep(): remove trailing OP_NULLs etc

rpeep(): remove trailing OP_NULLs etc

Perl_rpeep() elides OP_NULLs etc in the middle of an op_next chain, but
not at the start or end. Doing it at the start is hard (and not addressed
here); doing it at the end is trivial, and it just looks like a mistake in
the original code (there since 1994) that was (incorrectly) worried about
following through a null pointer.