Fix latent crash in do_text_output_multiline().

do_text_output_multiline() would fail (typically with a null pointer
dereference crash) if its input string did not end with a newline.  Such
cases do not arise in our current sources; but it certainly could happen
in future, or in extension code's usage of the function, so we should fix
it.  To fix, replace "eol += len" with "eol = text + len".

While at it, make two cosmetic improvements: mark the input string const,
and rename the argument from "text" to "txt" to dodge pgindent strangeness
(since "text" is a typedef name).

Even though this problem is only latent at present, it seems like a good
idea to back-patch the fix, since it's a very simple/safe patch and it's
not out of the realm of possibility that we might in future back-patch
something that expects sane behavior from do_text_output_multiline().

Per report from Hao Lee.

Report: <CAGoxFiFPAGyPAJLcFxTB5cGhTW2yOVBDYeqDugYwV4dEd1L_Ag@mail.gmail.com>

src/backend/executor/execTuples.c

@@ -1325,33 +1325,32 @@ do_tup_output(TupOutputState *tstate, Datum *values, bool *isnull)
  * Should only be used with a single-TEXT-attribute tupdesc.
  */
 void
-do_text_output_multiline(TupOutputState *tstate, char *text)
+do_text_output_multiline(TupOutputState *tstate, const char *txt)
 {
 	Datum		values[1];
 	bool		isnull[1] = {false};
 
-	while (*text)
+	while (*txt)
 	{
-		char	   *eol;
+		const char *eol;
 		int			len;
 
-		eol = strchr(text, '\n');
+		eol = strchr(txt, '\n');
 		if (eol)
 		{
-			len = eol - text;
-
+			len = eol - txt;
 			eol++;
 		}
 		else
 		{
-			len = strlen(text);
-			eol += len;
+			len = strlen(txt);
+			eol = txt + len;
 		}
 
-		values[0] = PointerGetDatum(cstring_to_text_with_len(text, len));
+		values[0] = PointerGetDatum(cstring_to_text_with_len(txt, len));
 		do_tup_output(tstate, values, isnull);
 		pfree(DatumGetPointer(values[0]));
-		text = eol;
+		txt = eol;
 	}
 }
 

src/include/executor/executor.h

@@ -284,7 +284,7 @@ typedef struct TupOutputState
 extern TupOutputState *begin_tup_output_tupdesc(DestReceiver *dest,
 						 TupleDesc tupdesc);
 extern void do_tup_output(TupOutputState *tstate, Datum *values, bool *isnull);
-extern void do_text_output_multiline(TupOutputState *tstate, char *text);
+extern void do_text_output_multiline(TupOutputState *tstate, const char *txt);
 extern void end_tup_output(TupOutputState *tstate);
 
 /*