For cases FF_LINESNGL and FF_LINEGLOB in pp_formline, take great care

For cases FF_LINESNGL and FF_LINEGLOB in pp_formline, take great care
to call get magic exactly once. This doesn't just avoid logical errors
with tied variables, it actually avoids panics (or worse) because a
pointer is retained to the string returned by the (first) call to
SvPV_const() for a future sv_chop(), and any future call to get magic
can invalidate the buffer that that pointer points to.
Also this removes the original crazy code that would set then reset the
length of a scalar, so as to only copy an initial portion of it, and
also copy the entire scalar including trailing newline (which might
require allocating more memory), only to immediately remove the newline
from the copy by reducing the length by one.

p4raw-id: //depot/perl@34139