utf8.c: Fix reference count in swash_to_invlist()

utf8.c: Fix reference count in swash_to_invlist()

The return SV* from this function was inconsistent in its reference
count.  In some cases it creates a new SV, which has a reference count
of 1, and in some cases it returned an existing SV without incrementing
the reference count.  If the caller thought it was getting its own copy,
and decremented the reference count, it could lead to a double free.