Perl_newASSIGNOP: fix on g++ builds

Perl_newASSIGNOP: fix on g++ builds

op.c: In function ‘OP* Perl_newASSIGNOP(PerlInterpreter*, I32, OP*, I32, OP*)’:
op.c:6605:15: error: jump to label ‘detach_split’ [-fpermissive]
               detach_split:
               ^
op.c:6631:22: note:   from here
                 goto detach_split;
                      ^
op.c:6584:30: note:   skips initialization of ‘PMOP* const pm’
                 PMOP * const pm = (PMOP*)right;