[perl #129106] Check for null PL_curcop in IN_LC()

[perl #129106] Check for null PL_curcop in IN_LC()

or, rather, in macros that it calls.

When exiting a string eval, the current cop may be freed, so PL_curcop
gets set to null.  With the -DC option, we may end up printfing NVs
during scope exit, so the locale macros used to make sure that the
locale is sane before converting the numbers to strings need to make
sure not to read PL_curcop->cop_hints when PL_curcop is null.

This used to crash with: ./miniperl -DC -e'eval "l/A"'

I’m not sure how to write a test for this, or even whether it’s worth
writing a test for -D, which often changes behaviour depending on
porters’ whims.