events: do not read garbage memory on empty control files

[pkova]

This is another one of the address sanitizer catches.

For whatever reason my directed messaging fakeship ended up in a situation with empty control.bin and memory.bin. When starting up this fakeship the address sanitizer blows up here:

vere/pkg/noun/events.c

Lines 459 to 467 in f5799a9

	pat_u->con_u = c3_malloc(len_w);
	if ( (len_w != read(pat_u->ctl_i, pat_u->con_u, len_w)) ||
	(len_w != sizeof(u3e_control) +
	(pat_u->con_u->pgs_w * sizeof(u3e_line))) )
	{
	c3_free(pat_u->con_u);
	pat_u->con_u = 0;
	return c3n;
	}

len_w is 0, malloc(0) is implementation defined but does give a non-null pointer on macos, which means that on line 462 we are reading random garbage.