[perl #119169] index with __PACKAGE__ for 2nd argument

The refactoring of fbm_compile in 66379c06cd to prepare for
c72a4eedff1 put in an SvIsCOW check before doing SvPV_force.  I sim-
ply changed the logic there so that SvPV_force would continue to have
its effect but without tripping up on read-only variables for which
SvPV_force would not need to make any changes anyway.

Now, if a COW scalar is read-only, we can’t call SvPV_force on it,
because it will die.

It turns out that we don’t actually need to call SvPV_force on COWs.
We can just go ahead and attach the BM magic and continue sharing
the buffer.