libctf, decl: avoid leaks of the formatted string on error

libctf, decl: avoid leaks of the formatted string on error

ctf_decl_sprintf builds up a formatted string in the ctf_decl_t's
cd_buf, but then on error this is hardly ever freed: we assume that
ctf_decl_fini frees it, but it leaks it instead.

Make it free it like any decent ADT should.

libctf/
	* ctf-decl.c (ctf_decl_fini): Free the cd_buf.
	(ctf_decl_buf): Once it escapes, don't try to free it later.