op.c:newSTATEOP: remove unnecessary null check

If PL_parser is null, then the else block will crash on
PL_parser->copline.
Test line breaks in PERL5DB

$ PERL5DB='sub DB::DB{} die' perl5.18.1 -dle 'print "ok"'
Died.
$ PERL5DB='sub DB::DB{}
die' perl5.18.1 -dle 'print "ok"'
ok

So everything after the line break is ignored.

In bleadperl:

$ PERL5DB='sub DB::DB{} die' ./perl -dle 'print "ok"'
Died.
$ PERL5DB='sub DB::DB{}
die' ./perl -dle 'print "ok"'
Died at -e line 1.

Now the whole environment variable is used.  This was probably changed
by commit 2179133.

Let’s call it a bug fix and test it.

(The ‘at -e line 1’ is not right, but the next commit will fix it.)
Fix line numbers with #! -d:foo and PERL5DB=$'\n'

Commit 2179133 inadvertently stopped the PERL5DB env var from being
truncated just before the first line break.  (I’m considering that
a bug fix.)

The result is that #!perl -d:foo will throw the line numbers off by
one, as will line breaks in PERL5DB:

$ PERL5DB='sub DB::DB{}'$'\n\n\n''' ./perl -dle 'warn "ok"'
ok at -e line 4.

#!perl -d:foo has thrown off line numbers since f0e67a1d291 in 5.12.

This commit fixes both, by storing the line number of #! -d or the
number 0 for -d on the command line in the new PL_parser->preambling
member, which now overrides any number in PL_curcop.