sed: fix breakage added by zero length match code

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

editors/sed.c

@@ -767,7 +767,7 @@ static int do_subst_command(sed_cmd_t *sed_cmd, char **line_p)
 		 * second is "" before "d", third is "" after "d".
 		 * Second match is NOT replaced!
 		 */
-		if (prev_match_empty || start != 0) {
+		if (prev_match_empty || start != 0 || start != end) {
 			dbg("inserting replacement at %d in '%s'", start, line);
 			do_subst_w_backrefs(line, sed_cmd->string);
 		} else {

testsuite/sed.tests

@@ -302,6 +302,10 @@ testing "sed zero chars match/replace advances correctly 2" \
 	"sed 's [^ .]* x g'" \
 	"x x.x\n" "" " a.b\n"
 
+testing "sed zero chars match/replace logic must not falsely trigger here" \
+	"sed 's/a/A/g'" \
+	"_AAA1AA\n" "" "_aaa1aa\n"
+
 # testing "description" "commands" "result" "infile" "stdin"
 
 exit $FAILCOUNT