libctf: work around an uninitialized variable warning

libctf: work around an uninitialized variable warning

GCC 11+ complains that sym is uninitialized in ctf_symbol_next.  It
isn't, but it's not quite smart enough to figure that out (it requires
domain-specific knowledge of the state of the ctf_next_t iterator
over multiple calls).

libctf/
	* ctf-lookup.c (ctf_symbol_next): Initialize sym to a suitable
	value for returning if never reset during the function.