[perl #70836] Fix err msg for unterminated here-doc in eval

[perl #70836] Fix err msg for unterminated here-doc in eval

$ perl -e '<<foo'
Can't find string terminator "foo" anywhere before EOF at -e line 1.

$ perl -e 'eval "<<foo"; die $@'
Can't find string terminator "
foo" anywhere before EOF at (eval 1) line 1.

An internal implementation detail is leaking out.

When the lexer happens to have a multiline string in its line buffer
(in a string eval or quoted construct), it looks for "\nfoo" instead
of "foo".  It was passing that same string to the error-reporting code
(S_missingterm), resulting in that extraneous newline.