[perl #123712] Fix /$a[/ parsing

The parser used to read more lines of input when parsing code interpo-
lated into quote-like operators, under some circumstance.  This would
result in code like this working, even though it should be a syn-
tax error:

s||${s/.*/|;
/s}Just another Perl hacker,
print

"${;s/.*/Just an";
other Perl hacker,
/s} die or return;
print

While this was harmless, other cases, like /$a[/<<a with no trailing
newline, would cause unexpected internal state that did not meet the
reasonable assumptions made by S_scan_heredoc, resulting in a crash.

The simplest fix is to modify the function that reads more input,
namely, lex_next_chunk, and prevent it from reading more lines of
input from inside a quote-like operator.  (The alternative would be to
modify all the calls to lex_next_chunk, and make them conditional.)
That breaks here-doc parsing for things like s//<<EOF/, but the
LEX_NO_TERM flag to lex_next_chunk is used only by the here-doc
parser, so lex_next_chunk can make an exception if it is set.

Author: Father Chrysostomos

t/op/lex.t

@@ -7,7 +7,7 @@ use warnings;
 
 BEGIN { chdir 't' if -d 't'; require './test.pl'; }
 
-plan(tests => 17);
+plan(tests => 18);
 
 {
     no warnings 'deprecated';
@@ -151,3 +151,11 @@ gibberish
       'gibberish containing &{+z} - used to crash [perl #123753]'
     );
 }
+
+fresh_perl_is(
+  '/$a[/<<a',
+  "syntax error at - line 1, next char ;\n" .
+  "Can't find string terminator \"a\" anywhere before EOF at - line 1.\n",
+   { stderr => 1 },
+  '/$a[/<<a with no newline [perl #123712]'
+);

toke.c

@@ -1252,7 +1252,7 @@ buffer has reached the end of the input text.
 */
 
 #define LEX_FAKE_EOF 0x80000000
-#define LEX_NO_TERM  0x40000000
+#define LEX_NO_TERM  0x40000000 /* here-doc */
 
 bool
 Perl_lex_next_chunk(pTHX_ U32 flags)
@@ -1266,6 +1266,8 @@ Perl_lex_next_chunk(pTHX_ U32 flags)
     bool got_some;
     if (flags & ~(LEX_KEEP_PREVIOUS|LEX_FAKE_EOF|LEX_NO_TERM))
 	Perl_croak(aTHX_ "Lexing code internal error (%s)", "lex_next_chunk");
+    if (!(flags & LEX_NO_TERM) && PL_sublex_info.sub_inwhat)
+	return FALSE;
     linestr = PL_parser->linestr;
     buf = SvPVX(linestr);
     if (!(flags & LEX_KEEP_PREVIOUS) &&