(perl #129125) copy form data if it might be freed

(perl #129125) copy form data if it might be freed

If the format SV also appeared as an argument, and the FF_CHOP
operator modified that argument, the magic and hence the compiled
format would be freed, and the next iteration of the processing
the compiled format would read freed memory.

Unlike my original patch this copies the formsv too, since
that is also stored in the magic, and is needed for presenting
literal text from the format.