asan: double free sb_kill

oss-fuzz hits a flaky crash with a double-free.  I think this is due
to gas static state not being reinitialised between testcases, a bug
with oss-fuzz not gas.  Anyway, this patch should avoid the problem.

	* input-scrub.c (input_scrub_push): Move init of sb_index..
	(input_scrub_reinit): ..to here.