memory leak in opts

[rls1004]

steps to reproduce :

$ gcc -pg -g tests/s-signal.c
$ uftrace record --disable --signal SIGUSR1@trace_on a.out
uftrace: install signal handlers to task 85074

=================================================================
==85073==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 17 byte(s) in 1 object(s) allocated from:
    #0 0x7fb2922a6f40 in realloc (/usr/lib/x86_64-linux-gnu/libasan.so.4+0xdef40)
    #1 0x55667c92f3a7 in strjoin /home/rls1004/uftrace/utils/utils.c:544
    #2 0x55667c8a9d9f in opt_add_string /home/rls1004/uftrace/uftrace.c:228
    #3 0x55667c8ad5aa in parse_option /home/rls1004/uftrace/uftrace.c:806
    #4 0x7fb290c93d0d in argp_parse (/lib/x86_64-linux-gnu/libc.so.6+0x12fd0d)
    #5 0x55667c8afb8d in main /home/rls1004/uftrace/uftrace.c:1092
    #6 0x7fb290b85b96 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)

SUMMARY: AddressSanitizer: 17 byte(s) leaked in 1 allocation(s).

opts->sig_trigger is allocated and not freed.

uftrace/uftrace.c

Lines 805 to 807 in 657d4be

	case OPT_signal:
	opts->sig_trigger = opt_add_string(opts->sig_trigger, arg);
	break;

uftrace/uftrace.c

Lines 1014 to 1026 in 657d4be

	static void free_opts(struct opts *opts)
	{
	free(opts->filter);
	free(opts->trigger);
	free(opts->sort_keys);
	free(opts->args);
	free(opts->retval);
	free(opts->tid);
	free(opts->event);
	free(opts->patch);
	free(opts->caller);
	free(opts->watch);
	}