Rationalise the logic in lex_start for creating a new temporary if the

Rationalise the logic in lex_start for creating a new temporary if the
passed in SV is read only or doesn't end in ';'. Also, allow a NULL
pointer to imply a zero length string, which saves creating a mortal
in pp_require, only for it to be ignored in favour of a new SV.

p4raw-id: //depot/perl@29647